XmlDeserializer shoud be parent class of XmlAttributeDeserializer

I think the only way how to manage code in this classes in inheritance. XmlDeserializer had some new features, but XmlAttributeDeserializer not. I think they shoud be same, but XmlAttributeDeserializer with using DeserializeAsAttribute. 

I splited tests XmlTest.cs into XmlDeserializerTest.cs and XmlAttributeDeserializerTest.cs for sure.