Add Range header modifier.

Add interceptor that modifies call "Range" header. It changes header
value to "bytes=value-".

subsonic-api/src/integrationTest/kotlin/org/moire/ultrasonic/api/subsonic/interceptors/BaseInterceptorTest.kt

@@ -0,0 +1,35 @@
+package org.moire.ultrasonic.api.subsonic.interceptors
+
+import okhttp3.Interceptor
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import org.junit.Before
+import org.junit.Rule
+import org.moire.ultrasonic.api.subsonic.rules.MockWebServerRule
+
+/**
+ * Base class for testing [okhttp3.Interceptor] implementations.
+ */
+abstract class BaseInterceptorTest {
+    @Rule @JvmField val mockWebServerRule = MockWebServerRule()
+
+    lateinit var client: OkHttpClient
+
+    abstract val interceptor: Interceptor
+
+    @Before
+    fun setUp() {
+        client = OkHttpClient.Builder().addInterceptor(interceptor).build()
+    }
+
+    /**
+     * Creates [Request] to use with [mockWebServerRule].
+     *
+     * @param additionalParams passes [Request.Builder] to add additionally required
+     * params to the [Request].
+     */
+    fun createRequest(additionalParams: (Request.Builder) -> Unit): Request = Request.Builder()
+            .url(mockWebServerRule.mockWebServer.url("/"))
+            .also { additionalParams(it) }
+            .build()
+}

subsonic-api/src/integrationTest/kotlin/org/moire/ultrasonic/api/subsonic/interceptors/RangeHeaderInterceptorTest.kt

@@ -0,0 +1,59 @@
+package org.moire.ultrasonic.api.subsonic.interceptors
+
+import okhttp3.Interceptor
+import okhttp3.mockwebserver.MockResponse
+import org.amshove.kluent.`should contain`
+import org.amshove.kluent.`should equal to`
+import org.amshove.kluent.`should not contain`
+import org.junit.Test
+
+/**
+ * Unit test for [RangeHeaderInterceptor].
+ */
+class RangeHeaderInterceptorTest : BaseInterceptorTest() {
+
+    override val interceptor: Interceptor
+        get() = RangeHeaderInterceptor()
+
+    @Test
+    fun `Should update uppercase range header`() {
+        mockWebServerRule.mockWebServer.enqueue(MockResponse())
+        val offset = 111
+        val request = createRequest {
+            it.addHeader("Range", "$offset")
+        }
+
+        client.newCall(request).execute()
+
+        val executedRequest = mockWebServerRule.mockWebServer.takeRequest()
+        executedRequest.headers.names() `should contain` "Range"
+        executedRequest.headers["Range"]!! `should equal to` "bytes=$offset-"
+    }
+
+    @Test
+    fun `Should not add range header if request doens't contain it`() {
+        mockWebServerRule.mockWebServer.enqueue(MockResponse())
+        val request = createRequest { }
+
+        client.newCall(request).execute()
+
+        val executedRequest = mockWebServerRule.mockWebServer.takeRequest()
+        executedRequest.headers.names() `should not contain` "Range"
+        executedRequest.headers.names() `should not contain` "range"
+    }
+
+    @Test
+    fun `Should update lowercase range header`() {
+        mockWebServerRule.mockWebServer.enqueue(MockResponse())
+        val offset = 51233
+        val request = createRequest {
+            it.addHeader("range", "$offset")
+        }
+
+        client.newCall(request).execute()
+
+        val executedRequest = mockWebServerRule.mockWebServer.takeRequest()
+        executedRequest.headers.names() `should contain` "Range"
+        executedRequest.headers["Range"]!! `should equal to` "bytes=$offset-"
+    }
+}

subsonic-api/src/main/kotlin/org/moire/ultrasonic/api/subsonic/SubsonicAPIClient.kt

@@ -8,6 +8,7 @@ import okhttp3.HttpUrl
 import okhttp3.OkHttpClient
 import okhttp3.ResponseBody
 import okhttp3.logging.HttpLoggingInterceptor
+import org.moire.ultrasonic.api.subsonic.interceptors.RangeHeaderInterceptor
 import org.moire.ultrasonic.api.subsonic.response.StreamResponse
 import org.moire.ultrasonic.api.subsonic.response.SubsonicResponse
 import retrofit2.Response
@@ -51,6 +52,7 @@ class SubsonicAPIClient(baseUrl: String,
                         .build()
                 chain.proceed(originalRequest.newBuilder().url(newUrl).build())
             }
+            .addInterceptor(RangeHeaderInterceptor())
             .also {
                 if (debug) {
                     it.addLogging()

subsonic-api/src/main/kotlin/org/moire/ultrasonic/api/subsonic/interceptors/RangeHeaderInterceptor.kt

@@ -0,0 +1,25 @@
+package org.moire.ultrasonic.api.subsonic.interceptors
+
+import okhttp3.Interceptor
+import okhttp3.Interceptor.Chain
+import okhttp3.Response
+
+/**
+ * Modifies request "Range" header to be according to HTTP standard.
+ *
+ * See [range rfc](https://tools.ietf.org/html/rfc7233).
+ */
+internal class RangeHeaderInterceptor : Interceptor {
+    override fun intercept(chain: Chain): Response {
+        val originalRequest = chain.request()
+        val headers = originalRequest.headers()
+        return if (headers.names().contains("Range")) {
+            val offset = "bytes=${headers["Range"]}-"
+            chain.proceed(originalRequest.newBuilder()
+                    .removeHeader("Range").addHeader("Range", offset)
+                    .build())
+        } else {
+            chain.proceed(originalRequest)
+        }
+    }
+}