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`, The `IsXml()` method has been replaced with `StartsWithXmlDeclaration()` and `IsWellFormedXml()`

Can't guarantee that `IsWellFormedXml` is fast though, but this check only happens in cases where previously it would just throw.
A better implementation on `IsWellFormedXml` is welcome!