Fix OIDC login regression

Requests may require a User-Agent header, and the change in #16972
accidentally removed it, resulting in requests getting rejected causing
login to fail.

synapse/http/client.py

@@ -390,6 +390,13 @@ async def request(
                         cooperator=self._cooperator,
                     )
 
+                # Always make sure we add a user agent to the request
+                if headers is None:
+                    headers = Headers()
+
+                if not headers.hasHeader("User-Agent"):
+                    headers.addRawHeader("User-Agent", self.user_agent)
+
                 request_deferred: defer.Deferred = treq.request(
                     method,
                     uri,