Fix regression with calling a drop with a empty string

lib/liquid/drop.rb

@@ -29,7 +29,7 @@ module Liquid
 
     # called by liquid to invoke a drop
     def invoke_drop(method_or_key)
-      if self.class.public_method_defined?(method_or_key.to_s.to_sym)
+      if method_or_key != '' && self.class.public_method_defined?(method_or_key.to_s.to_sym)
         send(method_or_key.to_s.to_sym)
       else
         before_method(method_or_key)

test/lib/liquid/drop_test.rb

@@ -155,4 +155,8 @@ class DropsTest < Test::Unit::TestCase
   def test_enumerable_drop_size
     assert_equal '3', Liquid::Template.parse( '{{collection.size}}').render('collection' => EnumerableDrop.new)
   end
+
+  def test_empty_string_value_access
+    assert_equal '', Liquid::Template.parse('{{ product[value] }}').render('product' => ProductDrop.new, 'value' => '')
+  end
 end # DropsTest