Assert that the session contains a given piece of data using a closure

Just like `$response->assertViewHas()` accepts a closure as a second parameter which is called to evaluate the contents of the view data, `$response->assertSessionHas()` could implement a similar behavior.

src/Illuminate/Foundation/Testing/TestResponse.php

@@ -861,6 +861,8 @@ public function assertSessionHas($key, $value = null)
                 $this->session()->has($key),
                 "Session is missing expected key [{$key}]."
             );
+        } elseif ($value instanceof Closure) {
+            PHPUnit::assertTrue($value($this->session()->get($key)));
         } else {
             PHPUnit::assertEquals($value, $this->session()->get($key));
         }