Outlook sets recipient type of Required attendees as MAPI_TO and
optional ones as MAPI_CC, so the fix is just to not only iterate
over the "to" list of recipients but also the "cc" one. We're
also setting the proper iCal value for this case (OPT-PARTICIPANT
instead of REQ-PARTICIPANT)
In [MS-OXOCAL] Section 2.2.4.10.7 says the recipient type is 0x01
as Required and 0x02 as Optional and other documents such as
[MS-OXCMSG] 2.2.3.1.2 indicates that MAPI_TO is 0x01 and MAPI_CC
is 0x02, that's why is stored in 'to' and 'cc' respectively.