cjk_decode.patch:
 - patch *all* CJK decoders to replace only the first byte of an invalid byte sequence (by U+FFFD). Example from the issue title: b'\xff\n'.decode('gb2312', 'replace') gives now '�\n' instead of just '�'
 - add at least one unit test for *each* path in the decoder (sometimes it was really hard to see how to go into a specific path, especially for the johab decoder!)
 - add testcases for euc_jis_2004 and shift_jis_2004
 - factorize "codec tests" (codectests) of all japanese EUC tests (euc_commontests)

Because I consider this issue as a bug, I would like to apply this patch to 2.7, 3.2 and 3.3.