handle 'upgrade' in comma-separated connection header

Firefox sends `keep-alive, upgrade`, not just `upgrade`,
which the proxy incorrectly turned into `close`

Author: minrk

Diff for: lib/http-proxy/common.js

@@ -53,7 +53,8 @@ common.setupOutgoing = function(outgoing, options, req, forward) {
   if (!outgoing.agent) {
     outgoing.headers = outgoing.headers || {};
     if (typeof outgoing.headers.connection !== 'string'
-        || outgoing.headers.connection.toLowerCase() !== 'upgrade'
+        || !outgoing.headers.connection.toLowerCase().split(',').some(
+          function (s) { if (s.trim() === 'upgrade') { return true; } })
        ) { outgoing.headers.connection = 'close'; }
   }