Fix offsetSet and offsetUnset in Collection

src/Collections/Collection.php

@@ -50,4 +50,17 @@ class Collection extends ImmutableCollection implements ICollection {
         }
         return $this;
     }
+
+    /**
+     * @inheritDoc
+     */
+    public function offsetSet ($offset, $value): void {
+        $this->set($offset, $value);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function offsetUnset ($offset): void {
+        $this->remove($offset);
+    }
 }

src/Collections/ImmutableCollection.php

@@ -128,13 +128,13 @@ class ImmutableCollection implements IImmutableCollection {
     /**
      * @inheritDoc
      */
-    public function offsetSet ($offset, $value) {
+    public function offsetSet ($offset, $value): void {
         throw new ImmutableException();
     }
     /**
      * @inheritDoc
      */
-    public function offsetUnset ($offset) {
+    public function offsetUnset ($offset): void {
         throw new ImmutableException();
     }