Add `:email_confirmation` to devise `insensitive_keys`

This fixes a bug where the email confirmation input value is case
sensative. For example, if the email input is `myemail@example.com` and
the email confirmation input is `Myemail@example.com` the form would
fail to submit.

config/initializers/devise.rb

@@ -36,7 +36,7 @@ Devise.setup do |config|
   # Configure which authentication keys should be case-insensitive.
   # These keys will be downcased upon creating or modifying a user and when used
   # to authenticate or find a user. Default is :email.
-  config.case_insensitive_keys = [:email]
+  config.case_insensitive_keys = [:email, :email_confirmation]
 
   # Configure which authentication keys should have whitespace stripped.
   # These keys will have whitespace before and after removed upon creating or