text: Title: Users keep getting repeated Activation Emails even though they are already active
  Body: Hi,
  
  I am using Djoser with the SimpleJWT plugin and for some reason my users get constant user Activation emails from my server even after they have activated in the past.  If they click on the activation link again it just says they have already activated.  
  
  Does it have something to do with updating a `User` that triggers this email again?  I would think the only time an activation email should be sent out is when a user is created and is not active.  Any ideas on why this would be happening?  Thanks!
  
  Djoser settings:
  ```python
  DJOSER = {
      'PASSWORD_RESET_CONFIRM_URL': 'users/reset-password/{uid}/{token}',
      'USERNAME_RESET_CONFIRM_URL': 'users/reset-username/confirm/{uid}/{token}',
      'ACTIVATION_URL': 'users/activate/{uid}/{token}',
      'SEND_ACTIVATION_EMAIL': True,
      'PASSWORD_CHANGED_EMAIL_CONFIRMATION': True,
      'USER_CREATE_PASSWORD_RETYPE': True,
      'SET_PASSWORD_RETYPE': True,
      'TOKEN_MODEL': None,
      'SERIALIZERS': {
          'user': 'restapi.serializers.CustomUserSerializer',
          'current_user': 'restapi.serializers.CustomUserSerializer',
      },
  }
  ```