Support any enumerable in mutlipart

[knightpp]

Closes #442

[knightpp]

@wojtekmach is there anything I can do to fix failing tests? I see one CI job passed.

[wojtekmach]

Thank you!

[wojtekmach]

@knightpp oh, I don't think 0 is a good default:

iex> url = "https://httpbin.org/anything"
iex> stream = Stream.cycle(["abc"]) |> Stream.take(3)
iex> Req.post!(url, form_multipart: [file: {stream, filename: "a.txt"}]).body["files"]
%{}
iex> Req.post!(url, form_multipart: [file: {stream, filename: "a.txt", content_length: 9}]).body["files"]
%{"file" => "abcabcabc"}

should I change this to size = Keyword.fetch!(options, :content_length)?

[wojtekmach]

FYI we should have kept this property from the above test:

assert size == byte_size(body)

i.e. the size will be used as the content-length of the entire HTTP request and the body will be that http request body`.