HTTP: Fix dissection of data following the HTTP Upgrade response

HTTP: Fix dissection of data following the HTTP Upgrade response

When a HTTP response includes HTTP headers and a subset of data for the
subdissector in the same frame, be sure to skip the HTTP headers for
reassembly of the latter data. Otherwise the HTTP headers will be
misinterpreted as the subprotocol (for example, WebSocket).

Bug: 16274
Change-Id: Ida6f6f2f7d0c463be2d498bfde5e8a9cd11a4b25
Reviewed-on: https://code.wireshark.org/review/35536
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Peter Wu <peter@lekensteyn.nl>