Make sure include tags are never blank

lib/liquid/tags/include.rb

@@ -38,6 +38,10 @@ module Liquid
     def parse(tokens)
     end
 
+    def blank?
+      false
+    end
+
     def render(context)
       partial = load_cached_partial(context)
       variable = context[@variable_name || @template_name[1..-2]]

test/liquid/blank_test.rb

@@ -1,5 +1,11 @@
 require 'test_helper'
 
+class BlankTestFileSystem
+  def read_template_file(template_path, context)
+    template_path
+  end
+end
+
 class BlankTest < Test::Unit::TestCase
   include Liquid
   N = 10
@@ -77,4 +83,11 @@ class BlankTest < Test::Unit::TestCase
     assert_template_result("  "*(N+1), wrap(' {{ "" }} '))
     assert_template_result(" "*(N+1), wrap("{% assign foo = ' ' %}{{ foo }}"))
   end
+
+  def test_include_is_blank
+    Liquid::Template.file_system = BlankTestFileSystem.new
+    assert_equal "foobar"*(N+1), Template.parse(wrap("{% include 'foobar' %}")).render()
+    assert_equal " foobar "*(N+1), Template.parse(wrap("{% include ' foobar ' %}")).render()
+    assert_equal "   ", Template.parse(" {% include ' ' %} ").render()
+  end
 end