Add failing tests for Issue #2204

For a point release upgrade, Devise should not throw an exception when trying
to downcase or strip globally configured keys. This would be a breaking
change in functionality and this test demonstrates the issue.
Call respond_to?(attr) before applying strip or downcase

This avoids calling strip and downcase on globally configured keys that may
not exist on all devise models. Fixes #2204, a regression introduced by
PR #2135. Also included a note about the intentional use of respond_to.
Merge pull request #2205 from latortuga/2204-regression-with-db-authenticatable

2204 regression with db authenticatable