BUG/MEDIUM: stream: don't ignore res.analyse_exp anymore

BUG/MEDIUM: stream: don't ignore res.analyse_exp anymore

It happens that no single analyser has ever needed to set res.analyse_exp,
so that process_stream() didn't consider it when computing the next task
expiration date. Since Lua actions were introduced in 1.6, this can be
needed on http-response actions for example, so let's ensure it's properly
handled.

Thanks to Nick Dimov for reporting this bug. The fix needs to be
backported to 1.7 and 1.6.