http2: Fix hanging paused stream

When NGHTTP2_ERR_PAUSE is returned from data_source_read_callback, we
might not process DATA frame fully.  Calling nghttp2_session_mem_recv()
again will continue to process DATA frame, but if there is no incoming
frames, then we have to call it again with 0-length data.  Without this,
on_stream_close callback will not be called, and stream could be hanged.

Bug: http://curl.haxx.se/mail/lib-2015-11/0103.html
Reported-by: Francisco Moraes