This patch by Martin v. Loewis changes the UTF-16 codec to only

write a BOM at the start of the stream and also to only read it as
BOM at the start of a stream.

Subsequent reading/writing of BOMs will read/write the BOM as ZWNBSP
character. This is in sync with the Unicode specifications.

Note that UTF-16 files will now *have* to start with a BOM mark
in order to be readable by the codec.