improve case comparision performace

[faisalmansoor]

Case specific queries does not take in to account database column collation, therefore, the generated queries force an index scan. I noticed the problem while using uniqueness validator which specifically either does a case sensitive or case insensitive comparison (I didn't find a way to make it use default database casing).

For a column with case insensitive collation, SQL_Latin1_General_CP1_CI_AS, uniqueness validator generates the following queries.

validates :title, uniqueness: true generates SELECT 1 AS one FROM [articles] WHERE [articles].[title] = N''t1'' COLLATE Latin1_General_CS_AS_WS ORDER BY [articles].[id] ASC OFFSET 0 ROWS FETCH NEXT 1 ROWS ONLY

while validates :title, uniqueness: {:case_sensitive => false} generates SELECT 1 AS one FROM [articles] WHERE LOWER([articles].[title]) = LOWER(N''t1'') ORDER BY [articles].[id] ASC OFFSET 0 ROWS FETCH NEXT 1 ROWS ONLY

both causes index scan, for a large table, this cause a lot io.

MySQL adapter detects column collation and generates queries which does not cause table scan, I ported the same behavior to sqlserver-adapter in this PR. It will be great if you guys can provide some feedback, if this approach seems ok then I'll add some tests also.

[faisalmansoor]

These are copied from https://github.com/rails/rails/blob/602ec7a4a21846db74f0371eb27784f19f80e28c/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L768

[metaskills]

Thanks so much for this work. I'll have master/v4.2.5 out in a few days or so.