https://github.com/sagebind/isahc does not send empty headers

README.md

When passing an empty header to isahc, it is not send inside the request.

It looks like this is the default behavior of curl, but it can be overriden:

$ curl -H 'X-with-value: a value which is preserved' -H 'X-without-value:' https://httpbin.org/get
{
  "args": {}, 
  "headers": {
    "Accept": "*/*", 
    "Host": "httpbin.org", 
    "User-Agent": "curl/7.65.3", 
    "X-Amzn-Trace-Id": "Root=1-5f157454-942b9837318cfebb67180e01", 
    "X-With-Value": "a value which is preserved"
  }, 
  "origin": "REDACTED", 
  "url": "https://httpbin.org/get"
}
# but if we add a `-H 'X-without-value;'`, it works
$ curl -H 'X-with-value: a value which is preserved' -H 'X-without-value:' -H 'X-without-value;' https://httpbin.org/get
{
  "args": {}, 
  "headers": {
    "Accept": "*/*", 
    "Host": "httpbin.org", 
    "User-Agent": "curl/7.65.3", 
    "X-Amzn-Trace-Id": "Root=1-5f157471-2b95cde35a4027182631ca8d", 
    "X-With-Value": "a value which is preserved", 
    "X-Without-Value": ""
  }, 
  "origin": "REDACTED", 
  "url": "https://httpbin.org/get"
}

Cargo.toml

[package]
name = "isahc-empty-headers"
version = "0.1.0"
authors = ["JustinRlle <[email protected]>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[[bin]]
name = "isahc-empty-headers"
path = "./main.rs"

[dependencies]
isahc = "0.9.5"

main.rs

use isahc::http::Request;
use std::io::Read;

fn main() {
    let req = Request::builder()
        .uri("https://httpbin.org/get")
        .header("X-with-value", "a value which is preserved")
        .header("X-without-value", "")
        .body(()).expect("failed to build request");
    println!("-> request:\n{:#?}", req);

    let res = isahc::send(req).expect("failed to send request");
    println!("-> response:\n{:#?}", res);
    let mut body = res.into_body();
    let mut buffer = Vec::new();
    body.read_to_end(&mut buffer).expect("failed to read body");
    let body = String::from_utf8(buffer).expect("body is invalid utf-8");
    println!("-> body:\n{}", body);
}

out.txt

-> request:
Request {
    method: GET,
    uri: https://httpbin.org/get,
    version: HTTP/1.1,
    headers: {
        "x-with-value": "a value which is preserved",
        "x-without-value": "",
    },
    body: (),
}
-> response:
Response {
    status: 200,
    version: HTTP/2.0,
    headers: {
        "date": "Mon, 20 Jul 2020 10:36:32 GMT",
        "content-type": "application/json",
        "content-length": "363",
        "server": "gunicorn/19.9.0",
        "access-control-allow-origin": "*",
        "access-control-allow-credentials": "true",
    },
    body: Body(363),
}
-> body:
{
  "args": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "deflate, gzip", 
    "Host": "httpbin.org", 
    "User-Agent": "curl/7.70.0-DEV isahc/0.9.5", 
    "X-Amzn-Trace-Id": "Root=1-5f1573b0-a7a7fdf43076e25ceaa9e2b0", 
    "X-With-Value": "a value which is preserved"
  }, 
  "origin": "REDACTED", 
  "url": "https://httpbin.org/get"
}