Make conversion of RevisionType explicit to avoid failure using Npgsql

Author: patearl

Src/NHibernate.Envers/Entities/RevisionTypeType.cs

@@ -30,7 +30,7 @@ public void NullSafeSet(DbCommand cmd, object value, int index, ISessionImplemen
 			} 
 			else 
 			{
-				cmd.Parameters[index].Value = value;
+				cmd.Parameters[index].Value = Convert.ToByte(value);
 			}
 		}