addition of api_download_dataset methods based on /api/histories... url

We propose this pull request to use the /api/histories/history_id/contents/dataset_id/display url rather than the /datasets/dataset_id/display url to download a file.
It enable a galaxy data download when the option require_login is True (no redirection towards http://galaxy/root)
We propose also to modify the methods _get and _get_retry of the Client class to allow the return of the request object and not only the request.json. Request object is used to write the content in the output file. Thus we take advantage of the bioblend request management including https urls. The actual download_dataset methods does not support the https.
Rather than adding `returnr`, I think it's better to change `_get_retry` to return the request object unconditionally and change client code accordingly. Since these methods are not public, we don't have to worry about backwards compatibility.