Revert collect method in interface, and gracefully skip if not implemented

Author: RWOverdijk

src/AssetManager/Resolver/AggregateResolver.php

@@ -60,6 +60,10 @@ public function collect()
         $collection = array();
 
         foreach ($this->queue as $resolver) {
+            if (!method_exists($resolver, 'collect') {
+                continue;
+            }
+
             $collection = array_merge($resolver->collect(), $collection);
         }
 

src/AssetManager/Resolver/ResolverInterface.php

@@ -12,10 +12,4 @@ interface ResolverInterface
      * @return  \Assetic\Asset\AssetInterface|null Asset instance when found, null when not.
      */
     public function resolve($path);
-
-    /**
-     * Collect all assets - used for warm up and for asset extraction
-     * @return \Assetic\Asset\AssetInterface[]
-     */
-    public function collect();
 }