fix: CO-823 FreeBusy missing uid parameter in CalDav request

[frisonisland]

There is an open bug about the Free/Busy response missing the UID parameter.
From RFC https://www.rfc-editor.org/rfc/rfc5545 the FreeBusy component, VFREEBUSSY, is required to have the uid.

Providing a null uid will result in the UID to not be added as I don't know where to get the value in all other cases.
But for this use case (CalDAV request) and due to the current bug I have decided to answer back the same UID that was provided in the request.
Reason is while debugging with a client (Thunderbird), I saw the UID in the request was different every time, plus, quoting from the RFC:

  When used to request free/busy time information, the "ATTENDEE"
  property specifies the calendar users whose free/busy time is
  being requested; the "ORGANIZER" property specifies the calendar
  user who is requesting the free/busy time; the "DTSTART" and
  "DTEND" properties specify the window of time for which the free/
  busy time is being requested; the "UID" and "DTSTAMP" properties
  are specified to assist in proper sequencing of multiple free/busy
  time requests.

and

  When used to reply to a request for free/busy time, the "ATTENDEE"
  property specifies the calendar user responding to the free/busy
  time request; the "ORGANIZER" property specifies the calendar user
  that originally requested the free/busy time; the "FREEBUSY"
  property specifies the free/busy time information (if it exists);
  and the "UID" and "DTSTAMP" properties are specified to assist in
  proper sequencing of multiple free/busy time replies.

About the tests: I have added a test about the response (behavioral)
and a couple of UT for the FreeBusy class. I didn't opt for a refactor or adding more tests, but this class is lacking a lot of them