BUG/MEDIUM: h1: Don't compare host and authority if only h1 headers are parsed

BUG/MEDIUM: h1: Don't compare host and authority if only h1 headers are parsed

When only request headers are parsed, the host header should not be compared to
the request authority because no start-line was parsed. Thus there is no
authority.

Till now this bug was hidden because this parsing mode was only used for the
response in the FCGI multiplexer. Since the HTTP checks refactoring, the request
headers may now also be parsed without the start-line.

This patch fixes the issue #610. It must be backported to 2.1.

(cherry picked from commit 7032a3fd0a71426d338190ec0eeb86f5c547c3dd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>