Literals with xsd:base64Binary datatype serialize incorrectly

[delocalizer]

Looking at rdflib.term source this appears to be because the lexical is taken as the decoded value, because no base64 encoder is defined. This is in contrast to xsd:hexBinary where the lexical is the encoded value.

Example:

>>> byts = b'foo'
>>> enc = hexlify(byts)
>>> lit = Literal(enc, datatype=XSD.hexBinary)
>>> print(lit.toPython(), lit)
b'foo', 666f6f   # expected

vs

>>> enc = b64encode(byts)
>>> lit = Literal(enc, datatype=XSD.base64Binary)
>>> print(lit.toPython(), lit)
b'foo', foo  # unexpected

This means for instance that Graph containing this literal in the o position actually serializes like:

@prefix ns1: <http://example.com/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

ns1:foo ns1:bar "foo"^^xsd:base64Binary,

when I expect:

@prefix ns1: <http://example.com/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

ns1:foo ns1:bar "Zm9v"^^xsd:base64Binary,

I can work around by using rdflib.term.bind to handle this datatype in a custom fashion, but I believe the fix is easy enough