use URI::DEFAULT_PARSER to get rid of deprecation warning

[sterlzbd]

This feels like a bit of a hack to get rid of the deprecation warning but seemed to be the simplest way. From what I can tell, URI.escape and URI.unescape have been deprecated because they shouldn't be used to escape actual URIs. In this case though we aren't using them to escape URIs and none of the recommended replacements (CGI.escape, URI.encode_www_form or URI.encode_www_form_component) allow only escaping specific characters which is the behavior we want.

I didn't add a spec because this is doesn't meaningfully change the code (URI.escape/URI.unescape already delegate to URI::DEFAULT_PARSER) and test_xml_name_not_screwed_up_by_xml_convention already tests this behavior.

Fixes #71