Several improvements about how XML is parsed.

[pitbulk]

• Rename safe_load_nokogiri into safe_load_xml.
• Refactor of safe_load_xml method.
• Improve how handle XML that contains syntax errors, added test.
• Prevent exceptions on .at_xpath called when the document was not loaded and user invoke elements accesor by checking the @document is not nil.

[pitbulk]

@johnnyshields, @ahacker1-securesaml any chance to check my PR? Thanks

[ahacker1-securesaml]

I think this is good (security-wise). I'm not really concerned about XML parsing, what I am most concerned about is the signature verification components.

Right before releases 2.x, I will raise my concerns about the SignedDocumentValidator class