vCard v2.1 -> v3.0 conversion: ENCODING=BASE64 should be translated to ENCODING=b

In v3.0, base64 encoding must be specified as `ENCODING=b` (case insensitive, I think).  See RFCs 2425, 2426.

Currently, upon conversion from v2.1 to v3.0, `ENCODING=BASE64` remains unchanged.
It should instead be converted to `ENCODING=b`.