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

See https://github.com/7digital/SevenDigital.Api.Wrapper/issues/189

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.
A better implementation on IsWellFormedXml is welcome!