When building a directive, the current sanity check prevents
a '>' to be used, which makes a DOCTYPE directive with an
internal subset be rejected. It is accepted by the parser
though, so what can be parsed cannot be encoded.

Improved the corresponding sanity check to mirror the behavior
of the parser (in the way it handles angle brackets, quotes,
and comments).

Fixes #10158

Change-Id: Ieffea9f870f2694548e12897f8f47babc0ea4414
Reviewed-on: https://go-review.googlesource.com/11630Reviewed-by: Russ Cox <rsc@golang.org>