Incorrectly typed dates in response types

[jhanc-helix]

All date fields I've encountered so far were incorrectly typed as Date instead of string. This includes InstancePoolSummary.timeCreated, Instance.timeCreated, WorkRequest.timeAccepted, WorkRequest.timeFinished, WorkRequest.timeStarted.

Seemingly the dates are serialized to ISO strings, but the codegen incorrectly types (probably all) date fields.

[jhanc-helix]

This applies #327 but the scope seems to be much wider.