Merge pull request android-async-http#650 from JedYu/master

remove the overwritten header in AsyncHttpClient  constructor

Author: smarek

library/src/main/java/com/loopj/android/http/AsyncHttpClient.java

@@ -255,6 +255,9 @@ public void process(HttpRequest request, HttpContext context) {
                                         header, clientHeaderMap.get(header),
                                         overwritten.getName(), overwritten.getValue())
                         );
+                        
+                        //remove the overwritten header
+                        request.removeHeader(overwritten);
                     }
                     request.addHeader(header, clientHeaderMap.get(header));
                 }