asyncHandler is not unwrapped before instanceof tests

[jozsefilles]

When using ThrottleRequestFilter, my custom TransferCompletionHandler is getting wrapped into an AsyncHandlerWrapper.

This later fails the type check in WriteListener's constructor so that onHeadersWritten() is never called:
this.progressAsyncHandler = future.getAsyncHandler() instanceof ProgressAsyncHandler ? (ProgressAsyncHandler<?>) future.getAsyncHandler() : null;

The same might apply to the NettyRequestSender's writeRequest method for testing against the types TransferCompletionHandler and AsyncHandlerExtensions.

[slandelle]

Dup of #1314, fixed in 2.1 branch (won't be in 2.0 one)

[jozsefilles]

Thanks for pointing into the right direction!