Fix client certificate support

Fixes: #1069

Author: WhyNotHugo

vdirsyncer/storage/dav.py

@@ -393,6 +393,7 @@ def __init__(
         ssl = prepare_verify(verify, verify_fingerprint)
         if ssl:
             self._settings["ssl"] = ssl
+            ssl.load_cert_chain(*self._settings["cert"])
 
         self.useragent = useragent
         self.url = url.rstrip("/") + "/"