Possible bug in `read_function`?

I was attempting to debug why https://github.com/rust-lang/crates.io/blob/120f8008fb08c21fc6cd239c1100692a0ff487e6/src/tests/record.rs#L310-L370 was returning "Problem's parsing json". Changing the URL to `http` and then inspecting the traffic w/ wireshark, it looks like the request body is never being sent. https://github.com/rust-lang/crates.io/blob/120f8008fb08c21fc6cd239c1100692a0ff487e6/src/tests/record.rs#L347-L349 is definitely being called, and the buffer passed in is being populated correctly. I'm not sure if this is a bug in crates.io or a bug in curl-rust.