[1.5.x] Required serializer to use bytes in loads/dumps

loads has no way to tell if it should provide text or bytes to the
serializer; bytes are more reasonnable for a serialized representation,
and are the only option for pickled data.

dumps can perform conversions on the value it receives from the
serializer; but for consistency it seems better to require bytes too.

The current code would cause an exception when loading pickled session
data. See next commit.

Also fixed a bug when checking for compressed data.

Backport of 58a086ac from master.