Removing the redundant requirement of having "fields property with empty array" for union types with no values.

Note : Values are optionals in the Discriminated Union
Reference : http://msdn.microsoft.com/en-us/library/vstudio/dd233226(v=vs.100).aspx
Fix InvalidCastException when deserializing a GUID from BSON.
-Fixed null reference error when using JEnumerable created with default constructor
-Tests
Merge branch 'master' of https://github.com/JamesNK/Newtonsoft.Json
-Bug fix
Merge pull request #380 from tamizhvendan/master

Removing the redundant requirement of having "fields property with empty array"
Merge pull request #383 from spadfield/fix/bson-guid-deserialization

Fix InvalidCastException when deserializing a GUID from BSON.
-Added xamarinios10 to .NET 4.0 portable build
-Added aspnetcore50 to .NET 4.5 portable build