Fix behavior of ::filter() operation.

spec/loophp/collection/CollectionSpec.php

@@ -467,7 +467,7 @@ public function it_can_explode(): void
             );
     }
 
-    public function it_can_filter_its_element(): void
+    public function it_can_filter(): void
     {
         $input = array_merge([0, false], range(1, 10));
 
@@ -488,10 +488,17 @@ public function it_can_filter_its_element(): void
             ->normalize()
             ->shouldIterateAs([1, 3, 5, 7, 9]);
 
-        $this
-            ->filter()
+        $this::with(['afooe', 'fooe', 'allo', 'llo'])
+            ->filter(
+                static function ($value) {
+                    return 0 === mb_strpos($value, 'a');
+                },
+                static function ($value) {
+                    return mb_strlen($value) - 1 === mb_strpos($value, 'o');
+                }
+            )
             ->normalize()
-            ->shouldIterateAs([]);
+            ->shouldIterateAs(['allo']);
     }
 
     public function it_can_flatten(): void

src/Operation/Filter.php

@@ -38,11 +38,13 @@ public function on(iterable $collection): Closure
         $callbacks = $this->callbacks;
 
         return static function () use ($callbacks, $collection): Generator {
-            $iterator = $collection;
+            $iterator = new IterableIterator($collection);
 
             foreach ($callbacks as $callback) {
-                yield from $iterator = new CallbackFilterIterator(new IterableIterator($iterator), $callback);
+                $iterator = new CallbackFilterIterator($iterator, $callback);
             }
+
+            yield from $iterator;
         };
     }
 }