BUG/MEDIUM: stream: Only allow L7 retries when using HTTP.

Only allow L7 retries when using HTTP, it only really makes sense for HTTP,
anyway, and as the L7 retries code assume the message will be HTX, it will
crash when used with mode TCP.
This should fix github issue #627.

This should be backported to 2.1 and 2.0.
diff --git a/src/stream.c b/src/stream.c
index f7ed26f..4fa41a7 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -2068,6 +2068,7 @@
 				si_b->state = SI_ST_REQ; /* new connection requested */
 				si_b->conn_retries = s->be->conn_retries;
 				if ((s->be->retry_type &~ PR_RE_CONN_FAILED) &&
+				    (s->be->mode == PR_MODE_HTTP) &&
 				    !(si_b->flags & SI_FL_D_L7_RETRY))
 					si_b->flags |= SI_FL_L7_RETRY;
 			}