fix: curl - avoid zero-sized HEAD bodies

[Fishrock123]

If there is no body then don't construct a zero-lengthed body.

Fixes: https://github.com/http-rs/surf/issues/321 Fixes: https://github.com/http-rs/surf/issues/218

[Fishrock123]

Confirmed working: https://github.com/http-rs/surf/issues/321#issuecomment-908750099

[Fishrock123]

@sagebind if you wouldn't mind, I'd like to know if this patch looks correct to you.

[sagebind]

This doesn't seem to be aiming to correct the right thing here. The problem is that Isahc understands three different scenarios:

• A request with a body of known size, including 0. (Body::from_reader_sized)
• A request with a body of unknown size. (Body::from_reader)
• A request without a body. (Body::empty, or equivalently ())

The problem is that Surf never indicates the third case to Isahc, only one of the first two cases.