Bug 1616345 - Refresh DNS on HTTP 421 r=dragana

Steps to reproduce:

I've got a group of web servers using DNS failover for high availability.
We return a HTTP 421 from the offline server, indicating to the browser
that it should attempt to reconnect.

Actual results:

Firefox acknowledges the 421 and closes connections, but continues to cache
the old DNS.

Expected results:

Firefox should clear DNS on a HTTP 421 as well, to improve the usability of
this status code for DNS failover.

Differential Revision: https://phabricator.services.mozilla.com/D65078

--HG--
extra : moz-landing-system : lando

netwerk/protocol/http/nsHttpTransaction.cpp

@@ -1739,6 +1739,7 @@ nsresult nsHttpTransaction::HandleContentStart() {
       case 421:
         LOG(("Misdirected Request.\n"));
         gHttpHandler->AltServiceCache()->ClearHostMapping(mConnInfo);
+        mCaps |= NS_HTTP_REFRESH_DNS;
 
         // retry on a new connection - just in case
         if (!mRestartCount) {