Add logic to handle single-use headers

The code now includes a mechanism to handle HTTP headers that are allowed to be used only once. This is done by storing such headers separately and checking their sense status before sending them again. This will prevent any issues arising from multiple instances of the same header.

https://stackoverflow.com/questions/15682496/will-duplicate-access-control-allow-origin-headers-break-cors

src/JK/RestServer/HeaderManager.php

@@ -13,6 +13,11 @@ final class HeaderManager
     /** @var array Colletion of HTTP headers */
     private $headers = array();
 
+    /** These headers are only once allowed */
+    private $singleHeader = [
+        "Access-Control-Allow-Origin"
+    ];
+
     /**
      * Sets some defaults
      */
@@ -121,7 +126,23 @@ public function sendAllHeaders()
         }
 
         foreach ($this->headers as $name => $value) {
-            header($name . ': ' . $value);
+            if (!$this->isSingleHeaderAndNotAlreadySent($name)) {
+                header($name . ': ' . $value);
+            }
         }
     }
+
+    /**
+     * Bugfix: Remove duplicates of headers which are required to be single
+     *
+     * https://stackoverflow.com/questions/15682496/will-duplicate-access-control-allow-origin-headers-break-cors
+     */
+    private function isSingleHeaderAndNotAlreadySent(string $name): bool
+    {
+        if (!in_array($name, $this->singleHeader)) {
+            return false;
+        }
+
+        return array_key_exists($name, array_keys(headers_list()));
+    }
 }