For URLs ending with /, the handler did not work, trying to append
".xml" to the path.

For instance, the "Share Memory by Communicating" returned the
following error:

        open /Users/francisco.souza/lib/go/doc/codewalk/sharemem/.xml: no such file or directory

R=adg, minux.ma
CC=golang-dev
https://golang.org/cl/5797065