chromewebdriver HEAD /shutdown return 404

```
$ chromedriver -v
ChromeDriver 2.20.353124 (035346203162d32c80f1dce587c8154a1efa0c3b)

$ chromedriver --port=12345
Starting ChromeDriver 2.20.353124
(035346203162d32c80f1dce587c8154a1efa0c3b) on port 12345

$ ruby -r 'net/http' -e 'Net::HTTP.start("127.0.0.1", 12345) {|h| puts h.head("/shutdown").code }'
404
```

rb/lib/selenium/webdriver/chrome/service.rb

@@ -78,7 +78,7 @@ def stop
             http.open_timeout = STOP_TIMEOUT / 2
             http.read_timeout = STOP_TIMEOUT / 2
 
-            http.head("/shutdown")
+            http.get("/shutdown")
           end
 
           @process.poll_for_exit STOP_TIMEOUT