__init__(): Fix an invariant, that the charset item in a chunk tuple

must be a Charset instance, not a string.  The bug here was that
self._charset wasn't being converted to a Charset instance so later
.append() calls which used the default charset would break.

_split(): If the charset of the chunk is '8bit', return the chunk
unchanged.  We can't safely split it, so this is the avenue of least
harm.