Port date parsing fix from swagger-codegen

[tovkal]

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Description

This is a fix that was originally merged into the swagger-codegen repo (original PR). A PR to bring the same fix to Swift 4 has being done to the openapi-generator repo.

[4brunu]

@tovkal Thanks for this PR. We have other two PR open related to dates.
Once I have some time, I will try to look into this 🙂

[4brunu]

@tovkal Thanks for your contribution.
We are working on trying to merge #3365 that will allow to define a custom JSONDecoder and a custom dateDecodingStrategy.
I think this is more flexible and will allow everyone to specify the behaviour that they want.
What do you think?

[wing328]

The fix is available in the newly-added swift5 generator so closing this one instead.