Fixes Xamarin Bug 16742:  XmlConvert.ToDateTime(string, XmlDateTimeSerializationMode) throws index out of bounds exception

Fix for bug + added tests for method
XmlConvert.ToDateTime(string, XmlDateTimeSerializationMode)
Additional tests for Xamarin Bug 16742 + style enhancements
Cleaned up the using statements in DateTime tests
Merge pull request #835 from HorstKakuschke/master

Fixes Xamarin Bug 16742:  XmlConvert.ToDateTime(string, XmlDateTimeSeria...