Multiple cookies for the same domain don't appear to get sent

[hoelzro]

Summary

If I have multiple cookies for a domain in a cookies file provided via -C, only one of them appears to get sent.

Reproduction

Create cookies.txt:

# Netscape HTTP Cookie File
# https://curl.se/docs/http-cookies.html
# This file was generated by libcurl! Edit at your own risk.

localhost	TRUE	/	FALSE	1793197913	test_cookie1	one
localhost	TRUE	/	FALSE	1793197913	test_cookie2	two

Run httpbin:

$ podman run -p 8000:80 kennethreitz/httpbin

Run monolith against that httpbin instance, and observe only a single value in the Cookie header:

$ monolith -C cookies.txt http://localhost:8000/headers
http://localhost:8000/headers
{
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, br, deflate", 
    "Cookie": "test_cookie2=two", 
    "Host": "localhost:8000", 
    "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:135.0) Gecko/20100101 Firefox/135.0"
  }
}

Suspected Cause

I believe the issue is the use of insert instead of append here:

monolith/src/session.rs

Line 152 in 8702e66

.insert(COOKIE, HeaderValue::from_str(&cookie_header_value).unwrap());