Loïc Riegel db261e3ed9
bugfix: str join outputs dates consistently (RFC2822 when possible) (#15629)
Closes #11265

# Description
``str join`` outputs dates just other commands: RFC2822 by default
otherwise RFC3339 for negative dates

# User-Facing Changes

```nushell
~> 2024-01-01
# => Mon, 1 Jan 2024 00:00:00 +0000 (a year ago)
~> '3000 years ago' | date from-human
# => -0975-04-23T20:57:07.217711700+02:00 (3000 years ago)
~> [ 2024-01-01 ] | str join
# => Mon, 1 Jan 2024 00:00:00 +0000
~> [ ('3000 years ago' | date from-human) ] | str join
# => -0975-04-23T20:57:56.221269600+02:00
```

# Tests + Formatting
OK
# After Submitting
Nothing
2025-04-24 08:32:29 -05:00
..

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