Added case StatementRef for conversion from jObject to StatementBase

If a statement which contains a statement reference as object is converted to a jObject and back, the statement reference is discarded.
Since a jObject returns null if a property is not set and a substatement must have an objectType i think this will work. It allows the substatement only if the current statement is no substatement.
[JObject.cs#L265](https://github.com/JamesNK/Newtonsoft.Json/blob/master/Src/Newtonsoft.Json/Linq/JObject.cs#L265)

I realized i have to change "SubStatement" in line 75 to "SubStatement.OBJECT_TYPE"