Increase test coverage

[tobixen]

There are still code sections that aren't exercised through the tests

[tobixen]

A coverage check was done prior to the 0.7-release, the stuff below remains, and a new check should be done prior to the 1.0-release.

Things that I won't fix - at least not before 0.7:

• hacks for python2-support is of course not covered when running tests with py3
• some weird corner cases reported in patch to support ctags and etags, and to allow response.reason to be optional to support Debian 9 requests library #81 and Necessary for iCloud.. #56 is not exercised by the test (will maybe help to fix Caldav test servers #45)
• other obscure error situations where an exception will be raised
• elements.ical
• DAVClient.mkcol
• CalendarObjectResource._create isn't 100% covered, but I've moved this to a separate issue Clean up CalendarObjectResource._create #96.

Things that should be looked into:

• DAVObject.canonical_url is not used anywhere, and can probably be removed, abandoned code? Should probably wait until 1.0 to preserve backward compatibility.
• _handle_prop_response: some weird corner cases not exercised