Ensure auth keys at the start of the i18n msg are properly cased

Otherwise if we humanized the whole string, it could cause us to change
the output of strings with periods and maybe other side-effects, since
we're changing the whole string from i18n.

This is safer as it only changes the first char of the translated
message, and only if it is a match with the first translated auth key,
so we can more safely humanize & downcase all auth keys to interpolate
in the message whenever needed.

Also add changelog for the change.

CHANGELOG.md

@@ -54,6 +54,7 @@
   * Fix passing `format` option to `devise_for` [#5732](https://github.com/heartcombo/devise/pull/5732)
   * Use `ActiveRecord::SecurityUtils.secure_compare` in `Devise.secure_compare` to match two empty strings correctly. [#4829](https://github.com/heartcombo/devise/pull/4829)
   * Respond with `401 Unauthorized` for non-navigational requests to destroy the session when there is no authenticated resource. [#4878](https://github.com/heartcombo/devise/pull/4878)
+  * Fix incorrect grammar of invalid authentication message with capitalized attributes, e.g.: "Invalid Email or password" => "Invalid email or password". (originally introduced by [#4014](https://github.com/heartcombo/devise/pull/4014), released on v4.1.0) [#4834](https://github.com/heartcombo/devise/pull/4834)
 
 
 Please check [4-stable](https://github.com/heartcombo/devise/blob/4-stable/CHANGELOG.md)

lib/devise/failure_app.rb

@@ -111,13 +111,16 @@ module Devise
         options[:scope] = "devise.failure"
         options[:default] = [message]
         auth_keys = scope_class.authentication_keys
-        keys = (auth_keys.respond_to?(:keys) ? auth_keys.keys : auth_keys).map { |key| scope_class.human_attribute_name(key).downcase }
-        options[:authentication_keys] = keys.join(I18n.t(:"support.array.words_connector"))
+        human_keys = (auth_keys.respond_to?(:keys) ? auth_keys.keys : auth_keys).map { |key|
+          scope_class.human_attribute_name(key).downcase
+        }
+        options[:authentication_keys] = human_keys.join(I18n.t(:"support.array.words_connector"))
         options = i18n_options(options)
-        translated_message = I18n.t(:"#{scope}.#{message}", **options)
-        # only call `#humanize` when the message is `:invalid` to ensure the original format
-        # of other messages - like `:does_not_exist` - is kept.
-        message == :invalid ? translated_message.humanize : translated_message
+
+        I18n.t(:"#{scope}.#{message}", **options).then { |msg|
+          # Ensure that auth keys at the start of the translated string are properly cased.
+          msg.start_with?(human_keys.first) ? msg.upcase_first : msg
+        }
       else
         message.to_s
       end