Add support for rack 1.5.x

Rack 1.5.0 added this change:

`Rack::Auth::AbstractRequest#scheme now yields strings, not symbols`

`rack-oauth2` checks the auth header scheme for the `:mac` or `:bearer` symbol. I just added a `to_s` on `@auth_header.scheme`  and changed the symbol to a string so it'll be backwards compatible.