Check for redirect when handling well-known URLs

Fix #584

vdirsyncer/storage/dav.py

@@ -140,12 +140,15 @@ def find_dav(self):
                 'GET', self._well_known_uri, allow_redirects=True,
                 headers=self.session.get_default_headers()
             )
-            return response.url
+            if response.history:
+                return response.url
         except (HTTPError, exceptions.Error):
-            # The user might not have well-known URLs set up and instead points
-            # vdirsyncer directly to the DAV server.
-            dav_logger.debug('Server does not support well-known URIs.')
-            return ''
+            pass
+
+        # The user might not have well-known URLs set up and instead points
+        # vdirsyncer directly to the DAV server.
+        dav_logger.debug('Server does not support well-known URIs.')
+        return ''
 
     def find_principal(self, url=None):
         if url is None: