Added Session::get_once, get and delete a value from the session data…

…, fixes #3012

classes/kohana/session.php

@@ -173,6 +173,24 @@ public function get($key, $default = NULL)
 		return array_key_exists($key, $this->_data) ? $this->_data[$key] : $default;
 	}
 
+	/**
+	 * Get and delete a variable from the session array.
+	 *
+	 *     $bar = $session->get_once('bar');
+	 *
+	 * @param   string  variable name
+	 * @param   mixed   default value to return
+	 * @return  mixed
+	 */
+	public function get_once($key, $default = NULL)
+	{
+		$value = $this->get($key, $default);
+
+		unset($this->_data[$key]);
+
+		return $value;
+	}
+
 	/**
 	 * Set a variable in the session array.
 	 *