Add return type hints (#6)

This adds a few return type hints to address deprecation notices and be forwards-compatible with future Symfony versions.

These changes are compatible even with PHP 7.2.

Author: mpdude

src/JMS/ObjectRouting/Metadata/ClassMetadata.php

@@ -39,7 +39,7 @@ public function merge(MergeableInterface $object): void
         $this->routes = array_merge($this->routes, $object->routes);
     }
 
-    public function serialize()
+    public function serialize(): string
     {
         return serialize(
             array(
@@ -49,7 +49,7 @@ public function serialize()
         );
     }
 
-    public function unserialize($str)
+    public function unserialize($str): void
     {
         list(
             $this->routes,

src/JMS/ObjectRouting/Twig/RoutingExtension.php

@@ -15,20 +15,20 @@ public function __construct(ObjectRouter $router)
         $this->router = $router;
     }
 
-    public function getFunctions()
+    public function getFunctions(): array
     {
         return [
             new TwigFunction('object_path', [$this, 'path']),
             new TwigFunction('object_url', [$this, 'url']),
         ];
     }
 
-    public function url($type, $object, array $extraParams = [])
+    public function url($type, $object, array $extraParams = []): string
     {
         return $this->router->generate($type, $object, true, $extraParams);
     }
 
-    public function path($type, $object, array $extraParams = [])
+    public function path($type, $object, array $extraParams = []): string
     {
         return $this->router->generate($type, $object, false, $extraParams);
     }