Allow "Authorization" as well as "AUTHORIZATION" for TokenType\Bearer

When using an Authorization header, all caps is enforced. It would be nice to support the alternative as demonstrated in the OAuth2 RFC.

```
GET /resource/1 HTTP/1.1
Host: example.com
Authorization: Bearer mF_9.B5f-4.1JqM
```

This PR allows the current "AUTHORIZATION" key to be used, but also allows support for "Authorization". I also added phpunit as a require-dev key and added tests for using the Authorization header in both formats.