Allow quoted single curly braces in variables.

test/liquid/variable_test.rb

@@ -201,4 +201,8 @@ class VariableResolutionTest < Test::Unit::TestCase
   def test_multiline_variable
     assert_equal 'worked', Template.parse("{{\ntest\n}}").render!('test' => 'worked')
   end
+
+  def test_quoted_single_curly_braces
+    assert_template_result "{user}", "{{ variable | prepend: '{' | append: '}' }}", 'variable' => 'user'
+  end
 end # VariableTest