Checklist

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

Minimal reproduction code and steps

1. Run http --print=B POST api.url.com field_name="Foo Bar"

Current result

usage:
    http [METHOD] URL [REQUEST_ITEM ...]

error:
    argument REQUEST_ITEM: 'Bar' is not a valid value

for more information:
    run 'http --help' or visit https://httpie.io/docs/cli

Expected result

{
    "field_name": "Foo Bar"
}