oc: Make sure we always use NSString keys in 'messages' dictionary

Different types for keys in Dictionaries leads to not finding
expected entry - we should make sure that everythime we set/get
entry from a dictionary, key object type is same
(cherry picked from commit e54151e104d351b4ffaecfa84d924c2d6d0d0315)
oc/MailFolder: Add more logging so we can trace invalid message kyes
(cherry picked from commit 220cd9c91c7428e2b45f5fba6ff640f50bf16e9f)
openchange: fix searching for numeric propertu values in mail message records

Dictionary keys are NSString. Thus we never find properties
with a number for name (like PidTagSubject for instance).
I can't tell how big the effect of this fix is.
So far, it fixed bumping the number of "LocalFreebusy" messages
created *evertime* outlook get started.
Merge pull request #44 from Zentyal/kamenim/zentyal-3.2-fix-restrictions-matching

zentyal 3.2 fix restrictions matching