Fix the logic for updating maildir flags.

I noticed this failing when offlineimap and dovecot accessed the same directory.
Dovecot adds lowercase flags for keywords the IMAP clients sends it, so I had a
file that was nomed like

....:2,a

With the old logic, infostr would be set to infomatch.group(1) which was ':2,a'.
We would then remove the '2,[A-Z]*' leaving only the 'a'. leving us in the end
with a new file named

.....:a2,....

If the old flags were in [A-Z], we should completly strip them. I looks like
the code intent was to merge them. That is what this change implements. We
convert the old flags into a set and merge that with the incoming flags.

After this change, the above case produces a file named

.......:2,...a