allow http headers to be set to support auto-translate

In order for MailChimp to utilize auto-translate (for example sending a confirmation email in German), it requires the Accept-Language header to be sent.  Allow headers to be specified in the params.

This would be most specifically used for the `subscribe` call, but should be useful for anywhere mailchimp will generate an email that could be auto-translated.

For example: 

`api.lists.subscribe(id: list_id, email: email, merge_vars: merge_vars, headers: {'Accept-Language' => request.env['HTTP_ACCEPT_LANGUAGE']})`