#17514 Allow strings as dates in typescript-node generator to fix error: data.toISOString() is not a function

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@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

I was having a problem with data generated from an untyped endpoint being passed into an endpoint that did have a date type for a field.

    TypeError: data.toISOString is not a function

      491 |             return transformedData;
      492 |         } else if (type === "Date") {
    > 493 |             return data.toISOString();
          |                         ^
      494 |         } else {
      495 |             if (enumsMap[type]) {
      496 |                 return data;

      at Function.serialize (../openapi/model/models.ts:493:25)
      at Function.serialize (../openapi/model/models.ts:510:69)
      at Function.serialize (../openapi/model/models.ts:510:69)
      at Function.serialize (../openapi/model/models.ts:510:69)

The fix coerces the string into a date before getting the ISO string to ensure uniformity. I'm open to just passing along the string as is too, but throwing on reasonably valid input doesn't make sense.