BUG/MINIR: proxy: Match on 429 status when trying to perform a L7 retry

BUG/MINIR: proxy: Match on 429 status when trying to perform a L7 retry

Support for 429 was recently added to L7 retries (0d142e075 "MINOR: proxy:
Add support of 429-Too-Many-Requests in retry-on status"). But the
l7_status_match() function was not properly updated. The switch statement
must match the 429 status to be able to perform a L7 retry.

This patch must be backported if the commit above is backported. It is
related to #2687.

(cherry picked from commit 62c9d51ca4d4f870723522b30d368d984f536e7e)
Signed-off-by: Willy Tarreau <w@1wt.eu>