http: ignore response body for HEAD requests

Responses to HEAD requests must not have a message body, so do not
attempt to use the Content-Length and similar headers that indicate a
non-empty message body in the response. Those point to the entity that
would be returned for non-HEAD requests.

Requests do not have request_method set, so do not limit desegmentation
by HEAD in that case. Verified by comparing the output of `tshark -O
http` and by looking in the Wireshark GTK UI.

Bug: 6064
Change-Id: I7f027a06d8480673ca9754037d080c3b25cc5816
Reviewed-on: https://code.wireshark.org/review/4357
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>