_handle_text(): Use _isstring() for stringiness test.

_handle_multipart(): Add a test before the ListType test, checking for
stringiness of the payload.  String payloads for multitypes means a
message with broken MIME chrome was parsed by a lax parser.  Instead
of raising a BoundaryError in those cases, the entire body is assigned
to the message payload (but since the content type is still
multipart/*, the Generator needs to be updated too).