Typescript-fetch: date is only converted to javascript date with runtime checks enabled

[kasperpeulen]

This make sure that date/datetime types are only typed as javascript date types with runtime checks are enabled. Otherwise it will be a string (the default).

WIP: Not much java experience, if someone with more experience could check this, would be greatly appreciated.

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[amakhrov]

Thanks for your contribution @kasperpeulen ! The PR looks good to me.
Could you please update the title to start with the changed generator name?

[typescript-fetch] ...pr name...