BUG/MINOR: stream: properly clear the previous error mask on L7 retries

BUG/MINOR: stream: properly clear the previous error mask on L7 retries

The cleanup of the previous error was incorrect on L7 retries, it would
OR two values while they're part of an enum, leaving some bits set.
Depending on the errors it was possible to occasionally see an internal
error ("I" flag) being logged.

This should be backported as far as 2.0, though the do_l7_retry() function
in in proto_htx.c in older versions.

(cherry picked from commit 75a4284babf50feecac17f9b32d07509b6c519ed)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>