tests: linkcheck: test server OKHandler: don't re-use do_HEAD request…

… handler during do_GET handler, because the former closes the response headers by calling 'self.end_headers'

tests/test_build_linkcheck.py

@@ -443,9 +443,10 @@ def do_HEAD(self):
         self.end_headers()
 
     def do_GET(self):
-        self.do_HEAD()
         content = b"ok\n"
+        self.send_response(200, "OK")
         self.send_header("Content-Length", str(len(content)))
+        self.end_headers()
         self.wfile.write(content)