Merge pull request #555 from boobooninja/date_filter

Merge pull request 555

lib/liquid/standardfilters.rb

@@ -310,7 +310,10 @@ module Liquid
     def to_date(obj)
       return obj if obj.respond_to?(:strftime)
 
-      obj = obj.downcase if obj.is_a?(String)
+      if obj.is_a?(String)
+        return nil if obj.empty?
+        obj = obj.downcase
+      end
 
       case obj
       when 'now'.freeze, 'today'.freeze

test/integration/standard_filter_test.rb

@@ -253,6 +253,8 @@ class StandardFiltersTest < Minitest::Test
 
     assert_equal nil, @filters.date(nil, "%B")
 
+    assert_equal '', @filters.date('', "%B")
+
     assert_equal "07/05/2006", @filters.date(1152098955, "%m/%d/%Y")
     assert_equal "07/05/2006", @filters.date("1152098955", "%m/%d/%Y")
   end