fix(url parser): add check for options as cb

Author: Jessica Lord

lib/url_parser.js

@@ -8,6 +8,9 @@ var ReadPreference = require('./read_preference'),
   dns = require('dns');
 
 module.exports = function(url, options, callback) {
+  if (typeof options === 'function') (callback = options), (options = {});
+  options = options || {};
+
   var result = parser.parse(url, true);
   if (result.protocol !== 'mongodb:' && result.protocol !== 'mongodb+srv:') {
     return callback(new Error('invalid schema, expected mongodb or mongodb+srv'));