Added filter date_to_rfc822

According to the W3C RSS Feed Validator, feeds must format dates as described
on RFC-822 to be valid. Refer to their site to get more info:
http://feedvalidator.org/docs/error/InvalidRFC2822Date.html

I also added a couple of missing unit tests to the other date filters

lib/jekyll/filters.rb

@@ -56,6 +56,20 @@ module Jekyll
       date.xmlschema
     end
 
+    # Format a date according to RFC-822
+    #
+    # date - The Time to format.
+    #
+    # Examples
+    #
+    #   date_to_rfc822(Time.now)
+    #   # => "Sun, 24 Apr 2011 12:34:46 +0000"
+    #
+    # Returns the formatted String.
+    def date_to_rfc822(date)
+      date.rfc822
+    end
+
     # XML escape a string for use. Replaces any special characters with
     # appropriate HTML entity replacements.
     #

test/test_filters.rb

@@ -13,6 +13,7 @@ class TestFilters < Test::Unit::TestCase
   context "filters" do
     setup do
       @filter = JekyllFilter.new
+      @sample_time = Time.parse "2013-03-27T11:22:33+00:00"
     end
 
     should "textilize with simple string" do
@@ -42,6 +43,22 @@ class TestFilters < Test::Unit::TestCase
       assert_equal "chunky, bacon, bits, and pieces", @filter.array_to_sentence_string(["chunky", "bacon", "bits", "pieces"])
     end
 
+    should "format a date with short format" do
+      assert_equal "27 Mar 2013", @filter.date_to_string(@sample_time)
+    end
+
+    should "format a date with long format" do
+      assert_equal "27 March 2013", @filter.date_to_long_string(@sample_time)
+    end
+
+    should "format a time with xmlschema" do
+      assert_equal "2013-03-27T11:22:33+00:00", @filter.date_to_xmlschema(@sample_time)
+    end
+
+    should "format a time according to RFC-822" do
+      assert_equal "Wed, 27 Mar 2013 11:22:33 +0000", @filter.date_to_rfc822(@sample_time)
+    end
+
     should "escape xml with ampersands" do
       assert_equal "AT&amp;T", @filter.xml_escape("AT&T")
       assert_equal "&lt;code&gt;command &amp;lt;filename&amp;gt;&lt;/code&gt;", @filter.xml_escape("<code>command &lt;filename&gt;</code>")