title: OkHttpClient throws exception after upgrading to OkHttp3
body: <p>I'm using following lines of code to add a default header to all of my requests sent using Retrofit2:</p>
  
  <pre><code>private static OkHttpClient defaultHttpClient = new OkHttpClient();
  static {
      defaultHttpClient.networkInterceptors().add(new Interceptor() {
          @Override
          public Response intercept(Chain chain) throws IOException {
              Request request = chain.request().newBuilder()
                      .addHeader("Accept", "Application/JSON").build();
              return chain.proceed(request);
          }
      });
  }
  </code></pre>
  
  <p>After upgrading retrofit to beta-3 version, I had to upgrade OkHttp to OkHttp3 also (actually I just changed package names from okhttp to okhttp3, the library is included inside retrofit). After that I get exceptions from this line:</p>
  
  <pre><code>defaultHttpClient.networkInterceptors().add(new Interceptor());
  </code></pre>
  
  <blockquote>
    <p>Caused by: java.lang.UnsupportedOperationException
                                                                      at java.util.Collections$UnmodifiableCollection.add(Collections.java:932)</p>
  </blockquote>
  
  <hr>
  
  <blockquote>
    <p>Caused by: java.lang.ExceptionInInitializerError</p>
  </blockquote>
  
  <hr>
  
  <p>What is the problem here?</p>