Add subroutine for normalizing `Authorization` header

Heroku API credentials can be specified in a number of ways:

    Authorization: Bearer <Token>
    Authorization: Basic <base64 encoded ":<token>">
    Authorization: Basic <base64 encoded "<email>:<token>">

This system adds normalization so at least the first two schemes above, which
are always equivalent, can be grouped so that if a 2FA token is procured for
one, it will stay valid for the other.