tests: linkcheck: capture_headers_handler: relocate the (client) head…

…er capture to before any server-side communication has been initiated by the handler

(cherry picked from commit 4d485ae)

tests/test_build_linkcheck.py

@@ -252,9 +252,9 @@ def do_HEAD(self):
             self.do_GET()
 
         def do_GET(self):
+            records.append(self.headers.as_string())
             self.send_response(200, "OK")
             self.end_headers()
-            records.append(self.headers.as_string())
     return HeadersDumperHandler