If there are nested <template> elements and the <template> node isn't in HTML namespace,
couldn't continue to parse documents correctly.
By this patch, it makes the <template> which is in math namespace be skipped on
resetting insertion mode.

Fixes golang/go#27702

Change-Id: I6eacdb98fe29eb3c61781afca5bc4d83e72ba4ed
Reviewed-on: https://go-review.googlesource.com/136875Reviewed-by: Nigel Tao <nigeltao@golang.org>