text: Title: Cleanup Usage of CIMultidict
  Body: While reviewing #2097, @ashleysommer pointed out how we have been using `header` which is a `CIMultidict` type. We have been using `header.get` instead of explicitly using either `getone` or `getall`
  
  In order to keep the original PR clean, we are opening a new tracker item to make sure we can cleanup the usage of this across the board in `sanic` codebase.
  
  1. Replace usage of `get` with a more suitable appropriate replacement of `getone` or `getall`
  
  