Merge pull request #592 in ZP/z-push from bugfix/ZP-1285-caldav-time-zone-incorrectly-converted to develop

* commit '5a8c3800':
  ZP-1285 Missed other instance of timezone lookup in Recurrence -- fixed. Note: Needs more eyeballs; this appears correct but the original code goes back a *long* way, and until recently I had not noted problems with time zone conversion.  Why it failed now, but not before, is not clear.
  ZP-1285 Fix the call to GMT conversion for DTSTART and DTEND conversion from DAVICAL events to ActiveSync format.  The timezone initialization passed was the ActiveSync one, which PHP does not know and thus failed to index properly for daylight savings time.  Replace with the check for a valid PHP time zone field from the Davical server and, if invalid, use the system default.