Only reset request headers after scenario if they were set

Setting manual request headers on a Chrome instance seems
somewhat unlikely, but it is exposed by the Mink interface
and implemented by the driver so they do need to be cleaned
up as part of ->reset() if present.

However, if the browser or devtools connection has crashed /
hit a problemn then calling Network.setExtraHttpHeaders can
fail in a way that masks and/or makes it harder to trace the
actual failure.

Therefore, avoid sending this call unless it is actually
required (e.g. some custom headers have been set since the
session started or was last reset).

src/ChromeDriver.php

@@ -239,8 +239,10 @@ public function reset()
         }
         $this->switchToWindow($this->main_window);
         $this->page->reset();
-        $this->request_headers = [];
-        $this->sendRequestHeaders();
+        if ($this->request_headers !== []) {
+            $this->request_headers = [];
+            $this->sendRequestHeaders();
+        }
     }
 
     /**