assert_xml_equal shows wrong source

When testing using the "old" `assert_xml_equal` I get an assertion error like the following.

```
3) Failure:
<bla bla message> [/home/nick/.rvm/gems/ruby-2.0.0-p195/gems/test_xml-0.1.6/lib/test_xml/assertions.rb:59]:
```

It should show the actual line in the failing test.
