MINOR: http: add the function "del-header" to the directives http-request and http-response

This patch permits to remove all HTTP request and response header fields
whose name is specified in <name>.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index f3fc1fc..53e9fb4 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -246,6 +246,7 @@
 	HTTP_REQ_ACT_AUTH,
 	HTTP_REQ_ACT_ADD_HDR,
 	HTTP_REQ_ACT_SET_HDR,
+	HTTP_REQ_ACT_DEL_HDR,
 	HTTP_REQ_ACT_REDIR,
 	HTTP_REQ_ACT_SET_NICE,
 	HTTP_REQ_ACT_SET_LOGL,
@@ -261,6 +262,7 @@
 	HTTP_RES_ACT_DENY,
 	HTTP_RES_ACT_ADD_HDR,
 	HTTP_RES_ACT_SET_HDR,
+	HTTP_RES_ACT_DEL_HDR,
 	HTTP_RES_ACT_SET_NICE,
 	HTTP_RES_ACT_SET_LOGL,
 	HTTP_RES_ACT_SET_TOS,