Update PUT URL to include p_guid

`master` PUT/POSTs to `path: /rd/reminders/tasks`, which - at least in my account - is 404ing with any set of arguments, including an unmodified Reminder. In a graphical browser, I can see that all PUTs and POSTs that I can generate (completing, creating, editing) go to `/pd/reminders/#{p_guid}` with no delimiters, like:

    /rd/reminders/1234b0e462ac58bbb43b00cb340816f42aad99f5d247d0aacf780d3620978e8f

My suspicion is that all of the Reminder-specific URLs should change to this, but I added it as a new method to be safe since:

* I don't know all intended uses of `put_reminder`/`post_reminder`, and
* My captures from a browser were far from exhaustive

I can say that calling this method/URL with a reminder that has `completed_date` set does complete the reminder and that as of today, that's what iCloud itself does in at least Chrome.