Added :default param for I18n.localize

Fixes #251

lib/i18n/backend/base.rb

@@ -62,6 +62,9 @@ def exists?(locale, key)
       # format string. Takes a key from the date/time formats translations as
       # a format argument (<em>e.g.</em>, <tt>:short</tt> in <tt>:'date.formats'</tt>).
       def localize(locale, object, format = :default, options = {})
+        if object.nil? && options.include?(:default)
+          return options[:default]
+        end
         raise ArgumentError, "Object must be a Date, DateTime or Time object. #{object.inspect} given." unless object.respond_to?(:strftime)
 
         if Symbol === format

lib/i18n/tests/localization/date.rb

@@ -51,6 +51,10 @@ def setup
           assert_nothing_raised { I18n.l(@date, options.freeze) }
         end
 
+        test "localize Date: given nil with default value it returns default" do
+          assert_equal 'default', I18n.l(nil, :default => 'default')
+        end
+
         test "localize Date: given nil it raises I18n::ArgumentError" do
           assert_raise(I18n::ArgumentError) { I18n.l(nil) }
         end

test/i18n_test.rb

@@ -269,6 +269,10 @@ def setup
     assert_raise(I18n::ArgumentError) { I18n.l nil }
   end
 
+  test "localize given nil and default returns default" do
+    assert_equal nil, I18n.l(nil, :default => nil)
+  end
+
   test "localize given an Object raises an I18n::ArgumentError" do
     assert_raise(I18n::ArgumentError) { I18n.l Object.new }
   end