title: Retrofit headers NullPointerException
body: <p>I have problem with retrofit and custom RequestInterceptor. I am trying to add header with content-type to every request i make, so i made custom RequestInterceptor like this: </p>
  
  <pre><code>new RestAdapter.Builder().
  setEndpoint(Configuration.BASE_ENDPOINT).
  setRequestInterceptor(new RequestInterceptor() {
                  @Override
                  public void intercept(RequestFacade request) {
                      request.addHeader("Content-type", "application/json");
                  }
              }).
  build();
  </code></pre>
  
  <p>but <strong><code>NullPointerException</code></strong> is thrown when i try to call some API. </p>
  
  <p>I traced down the issue to RequestBuilder class to this line: </p>
  
  <pre><code>headers.add(new Header("Content-Type", contentTypeHeader));
  </code></pre>
  
  <p><strong>Error :</strong></p>
  
  <pre><code> on line 344 : ArrayList headers is null. 
  </code></pre>
  
  <p>Do you know what am I doing wrong ? I used this exact code in other project and it was working just fine. </p>
  
  <p>Thanks</p>