I think that is because you can’t just specify a url for a file in faraday, it has to be read in as a file first, or something like that.
However, you can use the discourse_api gem to update an avatar by specifying a url:
client.update_avatar(
username: "norbert6",
url: "http://vignette1.wikia.nocookie.net/despicableme/images/1/15/Jerryindespicableme.png"
)