[5.4] add default param to collection's when() method (#17941)

* add optional $default param to when() to bring it in line with eloquent's when() method

* update collection tests for when() default param

* styleci fix

* add $default param to docblock

src/Illuminate/Support/Collection.php

@@ -313,12 +313,15 @@ public function filter(callable $callback = null)
      *
      * @param  bool  $value
      * @param  callable  $callback
+     * @param  callable  $default
      * @return mixed
      */
-    public function when($value, callable $callback)
+    public function when($value, callable $callback, callable $default = null)
     {
         if ($value) {
             return $callback($this);
+        } elseif ($default) {
+            return $default($this);
         }
 
         return $this;

tests/Support/SupportCollectionTest.php

@@ -1914,6 +1914,19 @@ public function testWhen()
 
         $this->assertSame(['michael', 'tom'], $collection->toArray());
     }
+
+    public function testWhenDefault()
+    {
+        $collection = new Collection(['michael', 'tom']);
+
+        $collection->when(false, function ($collection) {
+            return $collection->push('adam');
+        }, function ($collection) {
+            return $collection->push('taylor');
+        });
+
+        $this->assertSame(['michael', 'tom', 'taylor'], $collection->toArray());
+    }
 }
 
 class TestSupportCollectionHigherOrderItem