BUG/MINOR: h1-htx: Remove flags about protocol upgrade on non-101 responses

BUG/MINOR: h1-htx: Remove flags about protocol upgrade on non-101 responses

It is possible to have an "upgrade:" header and the corresponding value in
the "connection:" header for a non-101 response. It happens for
426-Upgrade-Required messages. However, on HAProxy side, a parsing error is
reported for this kind of message because no websocket key header
("sec-websocket-accept:") is found in the response.

So a possible fix could be to not perform this test for non-101
responses. However, having flags about protocol upgrade on this kind of
response could lead to other bugs. Instead, corresponding flags are
removed. Thus, during the H1 response post-parsing, H1_MF_CONN_UPG and
H1_MF_UPG_WEBSOCKET flags are removed from any non-101 response.

This patch should fix the issue #1997. It must be backported as far as 2.4.