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.

Author: carlosantoniodasilva

lib/devise/failure_app.rb

@@ -111,13 +111,16 @@ def i18n_message(default = nil)
         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