Recent breaking base64 change

[NathanBaulch]

The strfmt.Base64 JSON marshalers were recently switched from base64.StdEncoding to base64.URLEncoding in f620df9. This breaks a Go client I'm working on which consumes a third party .net service.
If this was intended then perhaps there needs to be a way to configure the preferred base64 encoder?

[fredbi]

@NathanBaulch URLEncoding has been here since inception.
What has changed in the BSON unmarshalling of Base64.

[casualjim]

I'll address it, I think it's a valid concern.