BUG/MINOR: lua/httpclient: missing free in hlua_httpclient_send()

In hlua_httpclient_send(), we replace hc->req.url with a new url.
But we forgot to free the original url that was allocated in
hlua_httpclient_new() or in the previous httpclient_send() call.

Because of this, each httpclient request performed under lua scripts would
result in a small leak. When stress-testing a lua action which uses httpclient,
the leak is clearly visible since we're leaking severals Mbytes per minute.

This bug was discovered by chance when trying to reproduce GH issue #2037.

It must be backported up to 2.5

src/hlua.c

@@ -7409,6 +7409,7 @@ __LJMP static int hlua_httpclient_send(lua_State *L, enum http_meth_t meth)
 
 	hlua_hc->sent = 0;
 
+	istfree(&hlua_hc->hc->req.url);
 	hlua_hc->hc->req.url = istdup(ist(url_str));
 	hlua_hc->hc->req.meth = meth;