Previously it generated them if they were given a value, but completely
ignored them if they were present in the string passed in to be parsed. Now
if the flag appears on a cookie, the corresponding Morsel key will reference a
True value. Other pre-existing behavior is retained in this maintenance
patch: if the source contains something like 'secure=foo', morsel['secure']
will return 'foo'. Since such a value doesn't round trip and never did (and
would be a surprising occurrence) a subsequent non-bug-fix patch may change
this behavior.
Inspired by a patch from Julien Phalip, who reviewed this one.
This also backs out the previous fixes for for #14360, #1717, and #16564.
Those bugs were actually caused by the fact that set_payload didn't decode to
str, thus rendering the model inconsistent. This fix does mean the data
processed by the encoder functions goes through an extra encode/decode cycle,
but it means the model is always consistent. Future API updates will provide
a better way to encode payloads, which will bypass this minor de-optimization.
Tests by Vajrasky Kok.
