crypto/x509: recognise ISO OID for RSA+SHA1

crypto/x509: recognise ISO OID for RSA+SHA1

For some reason, ISO decided to duplicate the OID for RSA+SHA1. Most
pertinantly, the makecert.exe utility on Windows is known to have used
this OID.

This change makes the ISO OID an alias for the normal one.

Change-Id: I60b76265bf1721282bdb0d5c99c98d227c18a878
Reviewed-on: https://go-review.googlesource.com/27394
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>