Sqlsrv fixes

One test ([ResultSet.fetchPairs()](https://github.com/nette/nette/blob/master/tests/Nette/Database/ResultSet.fetchPairs%28%29.phpt)) still failes with `Fatal error: Cannot access empty property`. MSSQL server returns empty column name for e.g. `id + 1` expression and [here](https://github.com/nette/nette/blob/256c6a3e2f3d7eb6d08513901e8cbcba019f753d/Nette/Database/ResultSet.php#L255) you get the fatal. Not sure how to fix it. Exception? Replace column name by number or e.g. `\x01`?