BUG/MINOR: http-fetch: Update method after a prefetch in smp_fetch_meth()

BUG/MINOR: http-fetch: Update method after a prefetch in smp_fetch_meth()

In smp_fetch_meth(), smp_prefetch_htx() function may be called to parse the
HTX message and update the HTTP transaction accordingly. In this case, in
smp_fetch_metch() and on success, we must update "meth" variable. Otherwise,
the variable is still equal to HTTP_METH_OTHER and the string version is
always used instead of the enum for known methods.

This patch must be backported as far as 2.0.