Previously, if there was a namespace defined on
a a>b tag, the namespace was ignored when
printing the parent elements. This fixes that,
and also fixes the racy behaviour of printerStack.trim
as discussed in https://go-review.googlesource.com/#/c/4152/10 .

Fixes #9796.

Change-Id: I75f97f67c08bbee151d1e0970f8462dd0f4511ef
Reviewed-on: https://go-review.googlesource.com/5910Reviewed-by: Nigel Tao <nigeltao@golang.org>