Fixed exception when sending empty credit card number

This fixes issue with exception being raised when sending empty credit card number.

```ruby
Recurly::Account.create billing_info: { number: '' }
ArgumentError: wrong number of arguments(2 for 1)
        from /var/proj/own-dev3/vendor/gems/ruby/1.9.1/gems/recurly-2.1.5/lib/recurly/xml.rb:39:in `[]'
```