Fixed bug where XmlDeserializer would not deserialize public nested types

The RestSharp XmlDeserializer only tries to deserialize types where
Type.IsPublic is true. For nested public classes, Type.IsPublic is
false, but Type.IsNestedPublic is true . Updated deserializer to check
for this condition
Merge pull request #582 from tomgallard/master

Fixed bug where XmlDeserializer would not deserialize public nested type