Test-code exercising non-base-URLs

Two Home Assistant users reported that they were unable to connect to their
caldav servers after upgrading HA.  Both of them got the same error message,
both of them had configured up a non-base-URL in the config.  Feeding DAVClient
with a calendar URL, principal URL or any other caldav URL should work and was
working with earlier versions of caldav, though it appears we have no specific
test code on that.

Now we have test code feeding the DAVClient with a principal URL and a calendar
URL.  Unfortunately, I haven't been able to reproduce the problem yet.

Updates #171
References #169
References home-assistant/core#65941
References home-assistant/core#65588

tests/test_caldav.py

@@ -1555,6 +1555,19 @@ def testRecurringDateSearch(self):
             assert_equal(len(r), 1)
         assert_equal(r[0].data.count("END:VEVENT"), 1)
 
+    def testOffsetURL(self):
+        """
+        pass an URL pointing to a calendar or a user to the DAVClient class,
+        and things should still work
+        """
+        urls = [ self.principal.url, self._fixCalendar().url ]
+        connect_params = self.server_params.copy()
+        connect_params.pop('url')
+        for url in urls:
+            conn = client(**connect_params, url=url)
+            principal = conn.principal()
+            calendars = principal.calendars()
+
     ## TODO: run this test, ref https://github.com/python-caldav/caldav/issues/91
     ## It should be removed prior to a 1.0-release.
     def testBackwardCompatibility(self):