text: Title: 'args' attribute of 'Command.CommandObj' seems not working
  Body: ## Context
  
  * Operating System: Windows 10
  * Python Version: 3.8.1
  * aiogram version: 2.12.1
  * aiohttp version: 3.7.3
  * uvloop version (if installed): 
  
  ## Expected Behavior
  
  `command.args` returns arguments of command
  
  ## Current Behavior
  
  `command.args` returns `None`
  
  ## Failure Information (for bugs)
  
  It seems the `args` attribute is not working.
  
  Digging into `dispatcher/filters/builtin.py`, I found nothing about setting this attribute, but after adding `args = text.split()[1]` and pass it to the construtor of `CommandObj`, it seems to work.
  
  I wonder if this is the intended behavior or am I getting it wrong?
  
  I would be glad to open a PR if it's a mistake :)
  
  ### Steps to Reproduce
  
  ```python
  @dp.message_handler(commands=['test'])
  async def test(message: Message, command: Command.CommandObj):
      print(command, command.args)
  ```
  
  Test command: `/test abc`
  
  ### Failure Logs
  
  Output: `Command.CommandObj(prefix='/', command='test', mention='') None`
  