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: https://github.com/nodejs/node/pull/4407
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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;