allow Token-Proxy override

* useful for nat scenarios

kamailio/pusher-role.cfg

@@ -72,7 +72,11 @@ route[PUSHER_PREPARE_PUSH_PAYLOAD]
     $var(TokenID) = $hdr(X-KAZOO-PUSHER-Token-ID);
     $var(TokenType) = $hdr(X-KAZOO-PUSHER-Token-Type);
     $var(TokenApp) = $hdr(X-KAZOO-PUSHER-Token-App);
+
     $var(TokenProxy) = $hdr(X-KAZOO-PUSHER-Token-Proxy);
+    #!ifdef PUSHER_TOKEN_PROXY
+    $var(TokenProxy) = $_s(PUSHER_TOKEN_PROXY);
+    #!endif
 
     ### token for fast reg ###
     $var(TokenReg) = $uuid(g);