validates_uniqueness_of is unicode unaware

Validate_uniqueness in case insensitive mode use downcase which is unicode-unaware. With an unicode aware database, lower(field) != downcased value so it fails to validate anything.

The patch is a simple one-liner.

In the test “Topic.find(t_utf8, :select => 'LOWER(title) AS title').title == "я тоже уникальный!"” checks unicode awareness of database. For example, In MySQL “alter database activerecord_unittest charset=utf8” make this condition true, and “alter database activerecord_unittest charset=latin1” make it false.