[9] RFR: 7145757: CertificateExtensions uses non-consistent key names

[Jason Uh]

Please review this fix, which changes the internal map in 
sun.security.x509.CertificateExtensions and CRLExtensions to always use 
the OID as the key.

As stated in the JBS issue:
> The sun.security.x509.CertificateExtensions class maintains a Map<String,Extension> map field to store all the extensions it manages. The key of map uses the name (say, "BasicConstraints") of the extension, or the OID when the type of the extension is unknown.

With this change, the map will consistently use the OID as the key.

webrev: http://cr.openjdk.java.net/~juh/7145757/00/
bug: https://bugs.openjdk.java.net/browse/JDK-7145757

Thanks,
Jason