FIX Allow to configure CurlLinkChecker request headers (#64)

* Allow to configure CurlLinkChecker request headers
* Make adding headers more explicit via array
* Remove extra character in comment
* Fix lint
* Align headers array with php docs

README.md

@@ -110,3 +110,19 @@ following config in config.yml.
 SilverStripe\ExternalLinks\Tasks\CurlLinkChecker::
   bypass_cache: 1
 ```
+
+## Headers
+
+You may want to set headers to be sent with the CURL request (eg: user-agent) to avoid website rejecting the request thinking it is a bot.
+You can set them with the following config in config.yml.
+
+```yaml
+# Headers
+SilverStripe\ExternalLinks\Tasks\CurlLinkChecker:
+  headers:
+    - 'user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0'
+    - 'accept-encoding: gzip, deflate, br'
+    - 'referer: https://www.domain.com/'
+    - 'sec-fetch-mode: navigate'
+    ...
+```

src/Tasks/CurlLinkChecker.php

@@ -31,6 +31,14 @@ class CurlLinkChecker implements LinkChecker
      */
     private static $bypass_cache = false;
 
+    /**
+     * Allow to pass custom header to be in CURL request
+     *
+     * @config
+     * @var array
+     */
+    private static $headers = [];
+
     /**
      * Return cache
      *
@@ -66,11 +74,19 @@ class CurlLinkChecker implements LinkChecker
         // No cached result so just request
         $handle = curl_init($href);
         curl_setopt($handle, CURLOPT_RETURNTRANSFER, true);
+        curl_setopt($handle, CURLOPT_CONNECTTIMEOUT, 5);
+        curl_setopt($handle, CURLOPT_TIMEOUT, 10);
         if ($this->config()->get('follow_location')) {
             curl_setopt($handle, CURLOPT_FOLLOWLOCATION, true);
         }
-        curl_setopt($handle, CURLOPT_CONNECTTIMEOUT, 5);
-        curl_setopt($handle, CURLOPT_TIMEOUT, 10);
+
+        // Add headers
+        $headers = (array) $this->config()->get('headers');
+        if (!empty($headers)) {
+            curl_setopt($handle, CURLOPT_HTTPHEADER, $headers);
+        }
+
+        // Retrieve http code
         curl_exec($handle);
         $httpCode = curl_getinfo($handle, CURLINFO_HTTP_CODE);
         curl_close($handle);
@@ -79,6 +95,7 @@ class CurlLinkChecker implements LinkChecker
             // Cache result
             $this->getCache()->set($cacheKey, $httpCode);
         }
+
         return $httpCode;
     }
 }