crypto/x509: fix behaviour of KeyUsageAny.
crypto/x509: fix behaviour of KeyUsageAny.

(Reporter wasn't able to provide a certificate chain that uses this
feature for testing.)

Fixes issue 6831

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/40340043

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go