[MINOR] http: add http_remove_header2() to remove a header value.

Calling this function after http_find_header2() automatically deletes
the current value of the header, and removes the header itself if the
value is the only one. The context is automatically adjusted for a
next call to http_find_header2() to return the next header. No other
change nor test should be made on the transient context though.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index f4a57a5..cfaf985 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -84,7 +84,7 @@
 int stats_check_uri_auth(struct session *t, struct proxy *backend);
 void init_proto_http();
 int http_find_header2(const char *name, int len,
-		      const char *sol, struct hdr_idx *idx,
+		      char *sol, struct hdr_idx *idx,
 		      struct hdr_ctx *ctx);
 void http_sess_log(struct session *s);
 void perform_http_redirect(struct session *s, struct stream_interface *si);