http2: fix dissection when using Upgrade

http2: fix dissection when using Upgrade

The fix for bug 11331 has as side-effect that the HTTP part of a
conversation is not dissected on the second pass.

Fix it by calling the HTTP2 dissector only when it was detected via
heuristics, and not via Upgrade (since that would be handled by the
http loop).

While at it, remove the use of tvb_new_subset_remaining since the
original tvb is not touched and move the comment about the proxy to the
right place.

Tested with the capture from Alexis (plain HTTP2 via Upgrade), the one
from bug 11331 (plain HTTP2 via heuristics) and a HTTP2 in SSL capture
(via heuristics).

Change-Id: Iead7682aa8d5114e4edcfd54eabcd0d659056cc1
Reviewed-on: https://code.wireshark.org/review/10541
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>