convert template source to string to ensure encoding validity

test/integration/template_test.rb

@@ -350,8 +350,9 @@ class TemplateTest < Minitest::Test
     assert_equal('Liquid syntax error: Invalid template encoding', e.message)
   end
 
-  def test_allows_nil_as_source
-    template = Template.parse(nil)
-    assert_equal('', template.render)
+  def test_allows_non_string_values_as_source
+    assert_equal('', Template.parse(nil).render)
+    assert_equal('1', Template.parse(1).render)
+    assert_equal('true', Template.parse(true).render)
   end
 end