Implement JsonSerializable

when beeing part of e.g. array the FullfilledPromise is not serialized, this will return the value now

src/Internal/FulfilledPromise.php

@@ -2,6 +2,7 @@
 
 namespace React\Promise\Internal;
 
+use JsonSerializable;
 use React\Promise\Promise;
 use React\Promise\PromiseInterface;
 use function React\Promise\enqueue;
@@ -11,7 +12,7 @@
 /**
  * @internal
  */
-final class FulfilledPromise implements PromiseInterface
+final class FulfilledPromise implements PromiseInterface , JsonSerializable
 {
     private $value;
 
@@ -77,4 +78,9 @@ public function always(callable $onFulfilledOrRejected): PromiseInterface
     public function cancel(): void
     {
     }
+
+    public function jsonSerialize()
+    {
+        return $this->value;
+    }
 }