fix duplicate content-type header in AHC client

Author: jenschude

commercetools/commercetools-async-http-client/src/main/java/com/commercetools/http/asynchttp/CtAsyncHttpClient.java

@@ -168,7 +168,7 @@ private Request asAhcRequest(final ApiHttpRequest request) {
                 mediaType = AsciiString
                         .of(Objects.requireNonNull(request.getHeaders().getFirst(ApiHttpHeaders.CONTENT_TYPE)));
             }
-            builder.addHeader(ApiHttpHeaders.CONTENT_TYPE, mediaType);
+            builder.setHeader(ApiHttpHeaders.CONTENT_TYPE, mediaType);
         });
         return builder.build();
     }