avoid casting exception when db provider returns ulong for GetLongScalar...

Since the GetLastInsertId refactor (commit https://github.com/ServiceStack/ServiceStack.OrmLite/commit/c4d584fcbeb17cf313396d2a71e19d2c512ba8d8), trying the use the GetLastInsertedId with MySql throws an invalid cast exception as MySql returns an ulong for the SelectIdentitySql)