Netty ssl parts

Parts were being sent twice when using HTTPS:
* once when setting the headers: https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.7.18/src/main/java/com/ning/http/client/providers/netty/NettyAsyncHttpProvider.java#L826
* once again when sending the request: https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.7.18/src/main/java/com/ning/http/client/providers/netty/NettyAsyncHttpProvider.java#L546

@jfarcand WDYT?