Add default filter to standard filters

lib/liquid/standardfilters.rb

@@ -245,6 +245,11 @@ module Liquid
       apply_operation(input, operand, :%)
     end
 
+    def default(input, default_value = "")
+      is_blank = input.respond_to?(:empty?) ? input.empty? : !input
+      is_blank ? default_value : input
+    end
+
     private
 
     def flatten_if_necessary(input)

test/liquid/standard_filter_test.rb

@@ -274,6 +274,15 @@ class StandardFiltersTest < Test::Unit::TestCase
     assert_template_result('abc',"{{ a | prepend: b}}",assigns)
   end
 
+  def test_default
+    assert_equal "foo", @filters.default("foo", "bar")
+    assert_equal "bar", @filters.default(nil, "bar")
+    assert_equal "bar", @filters.default("", "bar")
+    assert_equal "bar", @filters.default(false, "bar")
+    assert_equal "bar", @filters.default([], "bar")
+    assert_equal "bar", @filters.default({}, "bar")
+  end
+
   def test_cannot_access_private_methods
     assert_template_result('a',"{{ 'a' | to_number }}")
   end