Change .wb_download to use WB path ID

[psanker]

Around March 2022, I discovered that the OSF API no longer accepts GUIDs for downloading files. Instead, the Waterbutler path must be used. This PR just swaps the fid parameter in .wb_download() to use as_wb_id(), a new generic that mirrors the as_id() pattern but instead uses get_meta(x, "attributes", "path").