Revert case sensitivity for email uniqueness

[sidonath]

Hi everyone and thanks for maintaining the gem ❤️

Unfortunately, we ran into an issue after we updated Clearance to the latest version: some of our requests started failing due to statement timeouts in the database.

After a closer inspection it turned out the problem was related to queries on the users table which were changed to query by LOWER(email) instead of email. Since we didn't have the index on LOWER(email), the queries for some users got significantly slower.

This is related to the change made in #889—case_sensitive setting of the uniqueness validator was set
to false to prevent deprecation warnings from Rails.

However, the setting should have been set to true to preserve backwards-compatibility.

The deprecation warning said:

To continue case sensitive comparison
on the :email attribute in User model, pass case_sensitive: true
option explicitly to the uniqueness validator.

The problem was mentioned in an old commit that originally made the switch from the explicit false to the implicit true:
56f0f04

If I might recommend pushing this change in a bugfix release and if case_sensitive: false is indeed desired, push that change in Clearance 3.0.

[rtymchyk]

This would explain why our end-to-end tests suddenly started seeing network timeouts with the latest version.

[MottiniMauro]

Hi @sidonath , thanks for the contribution! We are merging to release it in the next bugfix release.