Add a few test cases to increase code coverage:

Add a few test cases to increase code coverage:

From:
 69.73% of 294 source lines executed in file ./Modules/_codecsmodule.c
 79.47% of 487 source lines executed in file Python/codecs.c
 78.45% of 3643 source lines executed in file Objects/unicodeobject.c

To:
 70.41% of 294 source lines executed in file ./Modules/_codecsmodule.c
 82.75% of 487 source lines executed in file Python/codecs.c
 80.76% of 3638 source lines executed in file Objects/unicodeobject.c

This actually unearthed a bug in the handling of None
values in PyUnicode_EncodeCharmap.