This allows subclassing TokenAuthentication and setting custom keyword,
thus allowing the Authorization header to be for example:
Bearer 956e252a-513c-48c5-92dd-bfddc364e812
It doesn't change the behavior of TokenAuthentication itself,
it simply allows to reuse the logic of TokenAuthentication without
the need of copy pasting the class and changing one hardcoded string.
Related: #4080
