title: Retrofit headers interceptor does not change header
body: <p>I have android app with few api calls. I noticed that every call has 
  <code>@Headers("Content-Type: application/json")</code> annotation in ApiService so I decided remove annotation and add header via interceptor to all requests:</p>
  
  <pre><code>    val headers = { chain: Interceptor.Chain -&gt;
          val request = chain.request().newBuilder()
                  .addHeader("Content-Type", "application/json")
                  .build()
          chain.proceed(request)
      }
  
      val logging = HttpLoggingInterceptor()
      logging.level = HttpLoggingInterceptor.Level.BODY
  
      val client = OkHttpClient.Builder()
              .addInterceptor(headers)
              .addInterceptor(logging)
              .build()
  
      val customGson = GsonBuilder()
              .registerTypeAdapter(NameValuesList::class.java, NamesValuesListConverter())
              .create()
  
      val retrofit = Retrofit.Builder()
              .baseUrl("http://www.$fullDomain")
              .addConverterFactory(GsonConverterFactory.create(customGson))
              .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
              .client(client)
              .build()
  
      service = retrofit.create(ApiService::class.java)
  </code></pre>
  
  <p>But after that server return error on api call.
  In logs I see that when I have explicit <code>@Headers()</code> annotation:</p>
  
  <pre><code>D/OkHttp: Content-Type: application/json
  </code></pre>
  
  <p>And after replace it with interceptor:</p>
  
  <pre><code>D/OkHttp: Content-Type: application/json; charset=UTF-8
  </code></pre>
  
  <p>I tried to change interceptor to this one:</p>
  
  <pre><code>    val headers = { chain: Interceptor.Chain -&gt;
          val request = chain.request().newBuilder()
                  .headers(Headers.of(mutableMapOf("Content-Type" to "test")))
                  .build()
          chain.proceed(request)
      }
  </code></pre>
  
  <p>But I still see this in log:</p>
  
  <pre><code>D/OkHttp: Content-Type: application/json; charset=UTF-8
  </code></pre>
  
  <p>So looks like my interceptor does not apply or overridden. How to fix it?</p>
  
  <p><strong>UPD</strong>. I found the reason: when I add GsonConverterFactory it automatically add header <code>Content-Type: application/json; charset=UTF-8</code>. Are there any way to avoid it without implementing custom ConverterFactory?</p>