Fixing bug in HTTP/2 ping handling.

Motivation:

The current HTTP/2 ping handling replies with an ack using the same
buffer as the received ping. It doesn't call retain(), however, which
causes a ReferenceCountException since the buffer ends up getting
released twice (once by the write and once by the decoder).

Modifications:

Modified AbstractHttp2ConnectionHandler to retain() the buffer. Added a
ping to Http2ConnectionRoundtripTest.stressTest() to verify the problem
and that this fixes it.

Result:

Ping should no longer cause an exception.