Add Accept header negotiation to relevant API endpoints (#337)

* start centralizing negotiation logic for API

* swagger document nodeinfo endpoint

* go fmt

* document negotiate function

* use content negotiation

* tidy up negotiation logic

* negotiate content throughout client api

* swagger

* remove attachment on Content

* add accept header to test requests

internal/api/client/media/mediacreate_test.go

@@ -149,6 +149,7 @@ func (suite *MediaCreateTestSuite) TestStatusCreatePOSTImageHandlerSuccessful()
 	}
 	ctx.Request = httptest.NewRequest(http.MethodPost, fmt.Sprintf("http://localhost:8080/%s", mediamodule.BasePath), bytes.NewReader(buf.Bytes())) // the endpoint we're hitting
 	ctx.Request.Header.Set("Content-Type", w.FormDataContentType())
+	ctx.Request.Header.Set("accept", "application/json")
 
 	// do the actual request
 	suite.mediaModule.MediaCreatePOSTHandler(ctx)