Currently any script tag is treated as a javascript container, although
<script type="text/template"> must not be. Check "type" attribute of
"script" tag. If it is present and it is not a JS MIME type, do not
transition to elementScript state.

Fixes #12149, where // inside text template was treated as regexp.
Fixes #6701

Change-Id: I8fc9e504f7280bdd800f40383c061853665ac8a2
Reviewed-on: https://go-review.googlesource.com/14336
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>