Default Accept-Language to I18n.default_locale

Currently AutoLocale tries to set `I18n.locale` to nil when the
Accept-Language header is missing, which leads to "nil is not a valid
locale" error when `I18n.config.enforce_available_locales` is
true (default).

This commit fixes the problem by defaulting the header value to
`I18n.default_locale`.

Author: knu

lib/http_accept_language/auto_locale.rb

@@ -11,7 +11,9 @@ module AutoLocale
     private
 
     def set_locale
-      I18n.locale = http_accept_language.compatible_language_from(I18n.available_locales)
+      hal = http_accept_language
+      hal.header ||= I18n.default_locale.to_s
+      I18n.locale = hal.compatible_language_from(I18n.available_locales)
     end
   end
 end

spec/auto_locale_spec.rb

@@ -6,19 +6,24 @@
 describe HttpAcceptLanguage::AutoLocale do
   let(:controller_class) do
     Class.new do
+      def initialize(header = nil)
+        super()
+        @header = header
+      end
+
       def self.before_filter(dummy)
         # dummy method
       end
 
       def http_accept_language
-        HttpAcceptLanguage::Parser.new("ja,en-us;q=0.7,en;q=0.3")
+        @http_accept_language ||= HttpAcceptLanguage::Parser.new(@header)
       end
 
       include HttpAcceptLanguage::AutoLocale
     end
   end
 
-  let(:controller) { controller_class.new }
+  let(:controller) { controller_class.new("ja,en-us;q=0.7,en;q=0.3") }
 
   context "available languages includes accept_languages" do
     before do
@@ -31,4 +36,19 @@ def http_accept_language
       expect(I18n.locale).to eq(:ja)
     end
   end
+
+  let(:no_accept_language_controller) { controller_class.new() }
+
+  context "default locale is ja" do
+    before do
+      I18n.default_locale = :ja
+      I18n.available_locales = [:en, :ja]
+    end
+
+    it "set the locale to default" do
+      no_accept_language_controller.send(:set_locale)
+
+      expect(I18n.locale).to eq(:ja)
+    end
+  end
 end