Prevent Readability::init() from resetting DOM if already set

Letting init() resetting the DOM may lead to unexpected behaviors in
libraries like Graby.

Related to https://github.com/j0k3r/graby/pull/392

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Readability.php

@@ -213,7 +213,9 @@ class Readability implements LoggerAwareInterface
      */
     public function init(): bool
     {
-        $this->loadHtml();
+        if (!isset($this->dom)) {
+            $this->loadHtml();
+        }
 
         if (!isset($this->dom->documentElement)) {
             return false;