Fixed sending of mock http error in kms_failpoint_server.py (#548)

Author: papafe

.evergreen/csfle/kms_failpoint_server.py

@@ -74,6 +74,8 @@ def _http_fail(self):
         global remaining_http_fails
         remaining_http_fails -= 1
         self.send_response(429)
+        self.end_headers()
+        print("mock http error")
 
     def do_POST(self):
         global remaining_http_fails