Fix for "Uncaught TypeError: Cannot read property '0' of undefined".

Hello,

A call to client.describe() fails for 'https://webservices.netsuite.com/wsdl/v2013_2_0/netsuite.wsdl' wsdl. The following mocha test can be used to reproduce the issue:

=================
var soap = require('..');
var assert = require('assert');


describe('NetSuite Service Manager', function() {

  it('should describe the wsdl', function(done) {
    this.timeout(0);
    soap.createClient('https://webservices.netsuite.com/wsdl/v2013_2_0/netsuite.wsdl', function(err, client) {
      assert.ok(client);
      console.log(client.describe());
      done();
    });
  });
});
=================

I made the required changes to fix this issue in wsdl.js. Please review.

Thanks,
Ankush