This isn't a DokuWiki bug, the error is on the client side.
DokuWiki shouldn't accept faulty requests (like double encoding).
The python module dose a base64 encode/decode on the data and passes it directly to the xmlrpclib.
according to the manual[1] the data must be wrapped into a Binary object.
[1]
http://docs.python.org/2/library/xmlrpclib.html#binary-objects