$ streamlink --json 'httpstream://localhost:8080 method=POST'
Traceback (most recent call last):
  File "/home/basti/venv/streamlink-39/bin/streamlink", line 33, in <module>
    sys.exit(load_entry_point('streamlink', 'console_scripts', 'streamlink')())
  File "/home/basti/repos/streamlink/src/streamlink_cli/main.py", line 1051, in main
    handle_url()
  File "/home/basti/repos/streamlink/src/streamlink_cli/main.py", line 595, in handle_url
    console.msg_json(
  File "/home/basti/repos/streamlink/src/streamlink_cli/console.py", line 79, in msg_json
    msg = dumps(out, cls=JSONEncoder, indent=2)
  File "/usr/lib/python3.9/json/__init__.py", line 234, in dumps
    return cls(
  File "/usr/lib/python3.9/json/encoder.py", line 201, in encode
    chunks = list(chunks)
  File "/usr/lib/python3.9/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/lib/python3.9/json/encoder.py", line 438, in _iterencode
    o = _default(o)
  File "/home/basti/repos/streamlink/src/streamlink_cli/utils/__init__.py", line 21, in default
    return obj.__json__()
  File "/home/basti/repos/streamlink/src/streamlink/stream/http.py", line 47, in __json__
    req = requests.Request(method=method, **valid_args(self.args))
TypeError: requests.models.Request() got multiple values for keyword argument 'method'

$ streamlink --json 'httpstream://localhost:8080 method=POST'
{
  "plugin": "http",
  "metadata": {
    "author": null,
    "category": null,
    "title": null
  },
  "streams": {
    "live": {
      "type": "http",
      "url": "https://localhost:8080/",
      "method": "POST",
      "headers": {
        "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:94.0) Gecko/20100101 Firefox/94.0",
        "Accept-Encoding": "gzip, deflate",
        "Accept": "*/*",
        "Connection": "keep-alive",
        "Content-Length": "0"
      },
      "body": null
    },
    "worst": {
      "type": "http",
      "url": "https://localhost:8080/",
      "method": "POST",
      "headers": {
        "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:94.0) Gecko/20100101 Firefox/94.0",
        "Accept-Encoding": "gzip, deflate",
        "Accept": "*/*",
        "Connection": "keep-alive",
        "Content-Length": "0"
      },
      "body": null
    },
    "best": {
      "type": "http",
      "url": "https://localhost:8080/",
      "method": "POST",
      "headers": {
        "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:94.0) Gecko/20100101 Firefox/94.0",
        "Accept-Encoding": "gzip, deflate",
        "Accept": "*/*",
        "Connection": "keep-alive",
        "Content-Length": "0"
      },
      "body": null
    }
  }
}