ensure string-format of auth-credentials

certbot_pdns/pdnsapi.py

         elif isinstance(verify_cert, str):          # alternative: path to local cert is given as string
             self.verify_cert = verify_cert          # see requests-documentation for more info
         
-    def set_http_auth(self, http_auth):             # credentials should be given as list containing two elements
-        if len(http_auth == 2):                     # first: username, second: password for http-basic auth
-            self.http_auth = (str(http_auth[0]), str(http_auth[1]))
+    def set_http_auth(self, http_auth):             # credentials should be given as list containing two string-elements
+        if len(http_auth) == 2:                     # first: username, second: password for http-basic auth
+            self.http_auth = (str(http_auth[0]), str(http_auth[1]))     # ensure right format of credentials
         
     def _query(self, uri, method, kwargs=None):
         headers = {