Revert "default rejectUnauthorized to true" (#496)

This reverts commit 2c55b27, which breaks websocket transport under node v0.10 (#494).

lib/socket.js

@@ -90,7 +90,7 @@ function Socket (uri, opts) {
   this.cert = opts.cert || null;
   this.ca = opts.ca || null;
   this.ciphers = opts.ciphers || null;
-  this.rejectUnauthorized = opts.rejectUnauthorized === undefined ? true : opts.rejectUnauthorized;
+  this.rejectUnauthorized = opts.rejectUnauthorized === undefined ? null : opts.rejectUnauthorized;
 
   // other options for Node.js client
   var freeGlobal = typeof global === 'object' && global;