Template lookup now respect default locale and I18n fallbacks.

Given the following templates:

    mailer/demo.html.erb
    mailer/demo.en.html.erb
    mailer/demo.pt.html.erb

Before this change for a locale that doesn't have its related file
the `mailer/demo.html.erb` will
be rendered even if `en` is the default locale.

Now `mailer/demo.en.html.erb` has precedence over the file without
 locale.

Also, it is possible to give a fallback.

    mailer/demo.pt.html.erb
    mailer/demo.pt-BR.html.erb

So if the locale is `pt-PT`, `mailer/demo.pt.html.erb` will be
 rendered given the right I18n fallback configuration.

Fixes #11884.

actionmailer/CHANGELOG.md

@@ -1 +1,24 @@
* Template lookup now respect default locale and I18n fallbacks.

  Given the following templates:

      mailer/demo.html.erb
      mailer/demo.en.html.erb
      mailer/demo.pt.html.erb

  Before this change for a locale that doesn't have its related file the `mailer/demo.html.erb` will
  be rendered even if `en` is the default locale.

  Now `mailer/demo.en.html.erb` has precedence over the file without locale.

  Also, it is possible to give a fallback.

      mailer/demo.pt.html.erb
      mailer/demo.pt-BR.html.erb

  So if the locale is `pt-PT`, `mailer/demo.pt.html.erb` will be rendered given the right I18n
  fallback configuration.

  *Rafael Mendonça França*

Please check [4-2-stable](https://github.com/rails/rails/blob/4-2-stable/actionmailer/CHANGELOG.md) for previous changes.

actionmailer/lib/action_mailer/base.rb

@@ -586,8 +586,6 @@ def process(method_name, *args) #:nodoc:
      }

      ActiveSupport::Notifications.instrument("process.action_mailer", payload) do
        lookup_context.skip_default_locale!

        super
        @_message = NullMail.new unless @_mail_was_called
      end

actionmailer/test/base_test.rb

@@ -353,10 +353,35 @@ def welcome
      assert_equal("text/plain", email.parts[0].mime_type)
      assert_equal("Implicit with locale PL TEXT", email.parts[0].body.encoded)
      assert_equal("text/html", email.parts[1].mime_type)
      assert_equal("Implicit with locale HTML", email.parts[1].body.encoded)
      assert_equal("Implicit with locale EN HTML", email.parts[1].body.encoded)
    end
  end

  test "implicit multipart with fallback locale" do
    fallback_backend = Class.new(I18n::Backend::Simple) do
      include I18n::Backend::Fallbacks
    end

    begin
      backend = I18n.backend
      I18n.backend = fallback_backend.new
      I18n.fallbacks[:"de-AT"] = [:de]

      swap I18n, locale: 'de-AT' do
        email = BaseMailer.implicit_with_locale
        assert_equal(2, email.parts.size)
        assert_equal("multipart/alternative", email.mime_type)
        assert_equal("text/plain", email.parts[0].mime_type)
        assert_equal("Implicit with locale DE-AT TEXT", email.parts[0].body.encoded)
        assert_equal("text/html", email.parts[1].mime_type)
        assert_equal("Implicit with locale DE HTML", email.parts[1].body.encoded)
      end
    ensure
      I18n.backend = backend
    end
  end


  test "implicit multipart with several view paths uses the first one with template" do
    old = BaseMailer.view_paths
    begin

actionmailer/test/fixtures/base_mailer/implicit_with_locale.de-AT.text.erb

@@ -0,0 +1 @@
Implicit with locale DE-AT TEXT

actionmailer/test/fixtures/base_mailer/implicit_with_locale.de.html.erb

@@ -0,0 +1 @@
Implicit with locale DE HTML

actionview/lib/action_view/lookup_context.rb

@@ -191,7 +191,6 @@ def normalize_name(name, prefixes) #:nodoc:

    def initialize(view_paths, details = {}, prefixes = [])
      @details, @details_key = {}, nil
      @skip_default_locale = false
      @cache = true
      @prefixes = prefixes
      @rendered_format = nil
@@ -213,12 +212,6 @@ def formats=(values)
      super(values)
    end

    # Do not use the default locale on template lookup.
    def skip_default_locale!
      @skip_default_locale = true
      self.locale = nil
    end

    # Override locale to return a symbol instead of array.
    def locale
      @details[:locale].first
@@ -233,7 +226,7 @@ def locale=(value)
        config.locale = value
      end

      super(@skip_default_locale ? I18n.locale : default_locale)
      super(default_locale)
    end

    # Uses the first format in the formats array for layout lookup.