Union accessors in C# should use generic type for the table

When accessing a union field, we should return the same object type
as was given to the method, i.e. the parameter should have a generic
type for any Table-derived type. This way, we do not need to make
superfluous casts (which also reduce type safety) like

var myUnionType = (MyUnionType)buff.GetUnionField(new MyUnionType());

when we can do just
var myUnionType = buff.GetUnionField(new MyUnionType());

Change-Id: Idac1b638e46cc50b1f2dc19f10741481202b1515

net/FlatBuffers/Table.cs

@@ -66,7 +66,7 @@ namespace FlatBuffers
         }
 
         // Initialize any Table-derived type to point to the union at the given offset.
-        protected Table __union(Table t, int offset)
+        protected TTable __union<TTable>(TTable t, int offset) where TTable : Table
         {
             offset += bb_pos;
             t.bb_pos = offset + bb.GetInt(offset);