html/template: lock in application/json as valid JS test

html/template: lock in application/json as valid JS test

CL https://go-review.googlesource.com/33899 added
application/json as a mimeType for valid JS. Let's
lock that fix in with a test.

Updates #18159

Change-Id: Ic4dfd8929aebfc5410f796688f081ca06630f672
Reviewed-on: https://go-review.googlesource.com/33901
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Nodir Turakulov <nodir@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>