MINOR: http-htx: condition uppercase acceptance in trailer names to HTTP_PF_UPCASE_OK

MINOR: http-htx: condition uppercase acceptance in trailer names to HTTP_PF_UPCASE_OK

The trailers parser currently rejects uppercase chars in the name.
Let's add a new flag to indicate that they are OK. This will be used
by HTTP/1. However this is not sufficient, we now need to use isteqi()
instead of isteq() to compare header names.

Neither H2 nor H3 need to pass that new flag, hence why the change is
minimal.