Fix registration in publisher, handle data_type attribute in Channel better

Not sure why/how the properties handling in registration wasn't an issue before, perhaps the Flight API had changed, but this fixes an issue where the resulting data from a publish request wasn't being unpacked into the properties dict properly.

Also in the process, the publish example in the README had an issue where the metadata accepted in Channels did not accept raw numpy types well, as they do not have the name attribute. Forcing a cast within the Channel class handles this well, since converting a raw numpy type gives a numpy.dtype object, but converting an already existing numpy.dtype object is a no-op.