[FIX] Erased timezone in $toMillis implementation

[LouisAuneau]

Issue

I noticed when converting an ISO 8601 date with a valid timezone, the timezone component is replaced by UTC, which corrupts the actual timestamp.

Reproduction:

from jsonata import Jsonata

expr = Jsonata('$fromMillis($toMillis("2025-01-01T01:00:00+01:00"))')
print(expr.evaluate({}))

You get: 2025-01-01T01:00:00.000Z
While the expected value should be: 2025-01-01T00:00:00.000Z

Proposed solution

Just adding a check to see if the extracted date is timezone aware before using dt.replace(tzinfo=datetime.timezone.utc).

Do not hesitate if you have question or suggestions for some changes. Thank you in advance!

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PS: Note none of the tests in jsonata base repository use another timezone than UTC, hence tests couldn't catch this case. I think it would be worth contributing to those tests as well. However the JS implementation does handle timezone changes correctly.