Removed leading '/' from resources URI
According to the API v3 documentation and other implementations, mandatory fields for creating "List Recipients" are ContactID and ListID
Forced Mailjet::Resource to send "Accept: application/json" and "Accept-Encoding: deflate" headers

* "Accept: application/json" is not really required but as the API returns JSON responses, it's not so bad to be consistent
* "Accept-Encoding: deflate" is here to avoid bogus GZipped responses from Mailjet server
Fixed https://github.com/mailjet/mailjet-gem/issues/26

RestClient::ResourceNotFound exceptions are handled by Mailjet::Connection and turned into Mailjet::ApiError exceptions