template.Clone() initialized template set incorrectly:
it didn't include itself.

* include itself in template set while cloning
* add a test

Fixes #12996

Change-Id: I932530e4f7f1bbebf833e12b000a5ce052bc9223
Reviewed-on: https://go-review.googlesource.com/16104Reviewed-by: Andrew Gerrand <adg@golang.org>