Sessions doesn't support multiple headers sharing the same name

[ducaale]

Checklist

• I've searched for similar issues.
• I'm using the latest version of HTTPie.

---

Minimal reproduction code and steps

Run http --offline : hello:world hello:people --session test

Current result

The header hello: world is not saved in the session file

$ cat ~/.httpie/sessions/localhost/test.json
{
    "__meta__": {
        "about": "HTTPie session file",
        "help": "https://httpie.io/docs#sessions",
        "httpie": "3.1.0"
    },
    "auth": {
        "password": null,
        "type": null,
        "username": null
    },
    "cookies": [],
    "headers": {
        "hello": "people"
    }
}

Expected result

Both hello: world and hello: people should be saved in the session file

---

Debug output

Please re-run the command with --debug, then copy the entire command & output and paste both below:

HTTPie 3.1.0
Requests 2.22.0
Pygments 2.7.2
Python 3.8.5 (default, May 27 2021, 13:30:53)
[GCC 9.3.0]
/usr/bin/python3
Linux 5.10.102.1-microsoft-standard-WSL2

<Environment {'apply_warnings_filter': <function Environment.apply_warnings_filter at 0x7ffa8d6de8b0>,
 'as_silent': <function Environment.as_silent at 0x7ffa8d6de790>,
 'colors': 256,
 'config': {'default_options': []},
 'config_dir': PosixPath('/home/ducaale/.httpie'),
 'devnull': <property object at 0x7ffa8d6d8900>,
 'is_windows': False,
 'log_error': <function Environment.log_error at 0x7ffa8d6de820>,
 'program_name': 'http',
 'quiet': 0,
 'stderr': <_io.TextIOWrapper name='<stderr>' mode='w' encoding='utf-8'>,
 'stderr_isatty': True,
 'stdin': <_io.TextIOWrapper name='<stdin>' mode='r' encoding='utf-8'>,
 'stdin_encoding': 'utf-8',
 'stdin_isatty': True,
 'stdout': <_io.TextIOWrapper name='<stdout>' mode='w' encoding='utf-8'>,
 'stdout_encoding': 'utf-8',
 'stdout_isatty': True}>

<PluginManager {'adapters': [],
 'auth': [<class 'httpie.plugins.builtin.BasicAuthPlugin'>,
          <class 'httpie.plugins.builtin.DigestAuthPlugin'>,
          <class 'httpie.plugins.builtin.BearerAuthPlugin'>],
 'converters': [],
 'formatters': [<class 'httpie.output.formatters.headers.HeadersFormatter'>,
                <class 'httpie.output.formatters.json.JSONFormatter'>,
                <class 'httpie.output.formatters.xml.XMLFormatter'>,
                <class 'httpie.output.formatters.colors.ColorFormatter'>]}>

>>> requests.request(**{'auth': None,
 'data': RequestJSONDataDict(),
 'headers': <HTTPHeadersDict('User-Agent': b'HTTPie/3.1.0', 'hello': b'world', 'hello': b'people')>,
 'method': 'get',
 'params': <generator object MultiValueOrderedDict.items at 0x7ffa8d346510>,
 'url': 'http://localhost'})

GET / HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost
User-Agent: HTTPie/3.1.0
hello: world
hello: people

Additional information, screenshots, or code examples

N/A