http: use self.keepAlive instead of self.options.keepAlive

In http.agent, all other options are directly accessed through
`self.` not `self.options`.

PR-URL: #4407
Reviewed-By: Brian White <[email protected]>
Reviewed-By: Fedor Indutny <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>

lib/_http_agent.js

@@ -58,7 +58,7 @@ function Agent(options) {
       if (req &&
           req.shouldKeepAlive &&
           !socket.destroyed &&
-          self.options.keepAlive) {
+          self.keepAlive) {
         var freeSockets = self.freeSockets[name];
         var freeLen = freeSockets ? freeSockets.length : 0;
         var count = freeLen;