BUG/MINOR: http: add-header: header name copied twice

The header name is copied two time in the buffer. The first copy is a printf-like
function writing the name and the http separators in the buffer, and the second
form is a memcopy. This seems to be inherited from some changes. This patch
removes the printf like, format.

This patch must be backported in 1.6 and 1.5 versions
diff --git a/src/proto_http.c b/src/proto_http.c
index 6686631..d32acb0 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3401,7 +3401,6 @@
 
 		case ACT_HTTP_SET_HDR:
 		case ACT_HTTP_ADD_HDR:
-			chunk_printf(&trash, "%s: ", rule->arg.hdr_add.name);
 			memcpy(trash.str, rule->arg.hdr_add.name, rule->arg.hdr_add.name_len);
 			trash.len = rule->arg.hdr_add.name_len;
 			trash.str[trash.len++] = ':';