Actual robust xml detection
on IApiResponseDetector, "IsXml" replaced with "StartsWithXmlDeclaration" and "IsWellFormedXml"
Can't guarantee that it's fast though, but this check only happens in cases where previously it would just throw
If there is an xml declaration, it should be a well-formed one
catch XmlException only
Merge pull request #190 from AnthonySteele/xml-detection

Xml detection when the xml does not have the (optional) declaration