(Bug 4875) Specialcase %BML::ML for post-request cleanup

`keys %BML::ML` gets us this in the logs:

        Can't locate object method "FIRSTKEY" via package "BML::ML"

So we just specialcase BML::ML and clear it after each request (instead
of trying to detect if it contains anything, etc)
Merge pull request #259 from afuna/bml-ml-firstkey-bug4875

(Bug 4875) Specialcase %BML::ML for post-request cleanup