Use authenticate_header method to get header prefix

[Rasmnev]

If authenticate_header method is used in line 38 it would be easier to use multiple prefixes by just subclassing TokenAuthentication and overriding that method

for example:

class BearerAuth(TokenAuthentication):
    def authenticate_header(self, request):
        return "Bearer"

REST_FRAMEWORK = {
  ...
  "DEFAULT_AUTHENTICATION_CLASSES": (
      "knox.auth.TokenAuthentication",
      "BearerAuth",
  ),
  ...
}

REST_KNOX = {
...
"AUTH_HEADER_PREFIX": "Token",
...
}

[Rasmnev]

Another possibility would be to add the keyword attribute to match DRF's TokenAuthentication