Only downcase first letter of each auth key, not the entire string

A previous change #4834 introduced a downcase call to each attribute, so
that it'd fix an invalid grammar issue on some languages like English
that were showing `Email` in the middle of flash message sentences.

However, it caused a bug with German which uses the word `E-Mail` and at
the beginning of the sentence, causing it to be converted to `E-mail`
incorrectly.

The fix here will only downcase the first char of each word, and convert
it back to upcase at the beginning of the sentence, which should work
for both the original fix (English message), and for the new bug (German
 message)

If we end up running into any more of these edge cases with the message,
we might roll it all back and provide a different set of interpolation
values for the original vs downcased translations, so people can use
what makes the most sense for each language without us having to
manually massage these strings.

Fixes #5820

Author: carlosantoniodasilva

lib/devise/failure_app.rb

@@ -112,7 +112,7 @@ def i18n_message(default = nil)
         options[:default] = [message]
         auth_keys = scope_class.authentication_keys
         human_keys = (auth_keys.respond_to?(:keys) ? auth_keys.keys : auth_keys).map { |key|
-          scope_class.human_attribute_name(key).downcase
+          scope_class.human_attribute_name(key).downcase_first
         }
         options[:authentication_keys] = human_keys.join(I18n.t(:"support.array.words_connector"))
         options = i18n_options(options)

test/failure_app_test.rb

@@ -215,6 +215,11 @@ def call_failure(env_params = {})
 
       assert_equal 'Email ou senha inválidos.', @request.flash[:alert]
       assert_equal 'http://test.host/users/sign_in', @response.second["Location"]
+
+      call_failure('warden' => OpenStruct.new(message: :invalid), 'warden.options' => { locale: :de })
+
+      assert_equal 'E-Mail oder Passwort ist ungültig.', @request.flash[:alert]
+      assert_equal 'http://test.host/users/sign_in', @response.second["Location"]
     end
 
     test 'uses the proxy failure message as string' do

test/support/locale/de.yml

@@ -0,0 +1,8 @@
+de:
+  activerecord:
+    attributes:
+      user:
+        email: E-Mail
+  devise:
+    failure:
+      invalid: "%{authentication_keys} oder Passwort ist ungültig."