crypto/x509: write exact BitLength in ASN.1 encoding for certificate KeyUsage

The encoded value of the certificate KeyUsage did contain additonal padding
that was not present with other certificate generators. According to ITU-T
X.690 the BitLength value should have no padding in a DER encoding.

See discussion:
https://groups.google.com/forum/#!topic/golang-nuts/dzaJ3hMpDcs

This CL has been discussed at: http://golang.org/cl/168990043

Change-Id: I1eff3f441b0566966a2d279631901ad9287c917d
Reviewed-on: https://go-review.googlesource.com/2255


Reviewed-by: Adam Langley <agl@golang.org>