Custom field compatibility with 2013_2 @script_id tag.

If you use the 2012_1 wsdl, you can call a custom field on a customer like this:
```ruby
customer.custom_field_list.custentity_example_field.value
```

When you change the wsdl to 2013_2, you will get undefined method custentity_example_field.  That is because the internal_id is now a numeric property and the old internal_id has been renamed to script_id.  This means that currently you have to get custom fields like this:
```ruby
customer.custom_field_list.custom_fields.select { |cf| cf.attributes['@script_id'.to_sym] == 'custentity_example_field' }.value
# or
customer.custom_field_list.send('509').value
```

This pull request addresses the problem by first trying to use script_id and then internal_id when a field is referenced, which means you can reference the field normally with the 2013_2 wsdl.