Make getHashForOID handle more algorithms

[misberner]

trafficstars

Specifically, SHA256 and every registered signature algorithms.

Shortcoming of the original version encountered when trying to parse PKCS7 data obtained via the Microsoft Azure VM Instance Metadata service (https://docs.microsoft.com/en-us/azure/virtual-machines/windows/instance-metadata-service)

[janisz]

@fullsailor will you merge it?