Merge pull request #311 from rogpeppe/019-nil-marshaler-test

add test for nil TestMarshaler

encode_test.go

@@ -24,6 +24,9 @@ var marshalTests = []struct {
 	{
 		nil,
 		"null\n",
+	}, {
+		(*marshalerType)(nil),
+		"null\n",
 	}, {
 		&struct{}{},
 		"{}\n",