Add support for id_token in request headers

[sauy7]

Currently, the fishbrain_id strategy only supports the id_token in the request params (query string). It would be useful if the id_token could be sent as a request header too.

See: https://github.com/fishbrain/omniauth-fishbrain/blob/master/lib/omniauth/strategies/fishbrain_id.rb#L57