Authentication and Progress handler overwriting each other in the git_fetch_options payload

[m3ta4a]

The Progress class seems currently incomplete but even with changes to make it functional I've run into an issue where the fetch options payload gets set twice, first with a Progress object followed by an Authentication object. This results in crashes in the progress handling, since the payload is no longer what was expected. I've begun working on a way to handle this but before I submit a pull request I thought I'd open an issue in case there was reasoning for this approach.