Fix FinderMethods#exists? sql generation to make picky sql servers happy

[iaddict]

Minimal change to query generation of FinderMethods#exists? that makes SQLServer and others happy that do not like columns without an alias.

Without this patch MS-SQLServer doesn't like the generated SQL and barks for FinderMethods#exists?, ie:

ActiveRecord::StatementInvalid: ActiveRecord::JDBCError: No column was specified for column 2 of 't'.: SELECT t.* FROM (SELECT ROW_NUMBER() OVER(ORDER BY huhu.id) AS _row_num, 1 FROM huhu WHERE huhu.[id] = 83287) AS t WHERE t._row_num BETWEEN 1 AND 1

[iaddict]

A test case has been added.

[mhfs]

@iaddict out of curiosity, since exists? is extensively tested, does the existent tests break against SQL Server?

[rafaelfranca]

This can't be merged automatically.

If you still have this issue, please rebase it and I'll glad to put this is in discussion with the Core Team.

[carlosantoniodasilva]

This seems to be recurrent, I've already closed 3 issues explaining the same problem, linked above. Unfortunately this pull request does not apply anymore, as @rafaelfranca pointed, so if you can bring it up-to-date, we can go ahead with it. Thanks