Add RFC 4791 MKCALENDAR method

Without it I get
```
ERROR ActionController::UnknownHttpMethod: MKCALENDAR, accepted HTTP methods are OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, CONNECT, PROPFIND, PROPPATCH, MKCOL, COPY, MOVE, LOCK, UNLOCK, VERSION-CONTROL, REPORT, CHECKOUT, CHECKIN, UNCHECKOUT, MKWORKSPACE, UPDATE, LABEL, MERGE, BASELINE-CONTROL, MKACTIVITY, ORDERPATCH, ACL, SEARCH, and PATCH
```