add support for subscribing to read-only public CALDAV calendars with no authentication

I'm trying to add a publicly view-able calendar URL made available via a fruux.com account. There is no username or password required to view this calendar, so I just put 'guest' in for the username and password. This yields an HTTP status code 405 Error message.

Please add support for subscribing to read-only public calendars that do not require any authentication.

Launchpad Details: [#LP1658819](https://bugs.launchpad.net/bugs/1658819) Seth - 2017-01-23 22:19:09 +0000

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