Fix fwrite POSIXct to write ISO-8601 datetimes

[aakritixyz]

• Added test for fwrite POSIXct writing to ISO-8601 format (UTC).
• Coerced POSIXt columns to double in fwrite for proper ISO formatting.
• Fixes issue where fwrite wrote POSIXct as numeric seconds instead of ISO strings.

[ben-schwen]

Please read our CONTRIBUTING guide and make yourself familiar how our tests in tests.Rraw work.

Also please link the corresponding issue #3535

In this case, the change might be only subtle such as making the rule system in whichWriter in fwriteR.c a bit smarter.