html/template: preserve attr in stateBeforeValue

html/template: preserve attr in stateBeforeValue

Context: #12149. The problem there is that contents of
<script type="text/template"> are treated as JS, and thus // is treated
as regexp.

Preserve context.attr while we are in the attribute, in particular in
stateBeforeValue, so we have attr when reading attr value.

Next CL will actually fix the bug.

Change-Id: I99add2237b0885ecdcc08b4f7c25d0af99173e53
Reviewed-on: https://go-review.googlesource.com/14335
Reviewed-by: Rob Pike <r@golang.org>