[Swift] handle decoding of unknown enum types if optional

Description

If I have a model class, say vehicle which has an enum 'type' which can have two values, 'car' and 'motorbike'. This class is generated with Swagger and instantiated from a JSON response received from my backend(using the codable protocol). Say I ship my App version 1.0 with this.

Later on I decide to add another vehicle type in my backend which will be handled in my app 1.1, 'bus', my app version is was released earlier will not be able to deserialize Vehicle. It will throw an error even if the type is not 'required':
dataCorrupted(Swift.DecodingError.Context(codingPath: ["type"], debugDescription: "Cannot initialize ModelType from invalid String value Something", underlyingError: nil))

Swagger-codegen version

swagger-codegen-cli-2.3.0-20171207.164637-326

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

It would be much nicer to have an option to deserialize optional unknown enum types to nil(like the option READ_UNKNOWN_ENUM_VALUES_AS_NULL in Jackson for Java) instead of throwing an error. This can be done by doing:

 type = try? container.decodeIfPresent(ModelType.self, forKey: "type")

instead of

type = try container.decodeIfPresent(ModelType.self, forKey: "type")

in the generated method of the class containing the enum

    public required init(from decoder: Decoder) throws 

Start by locating the Swift generator template that emits the model's required init(from decoder:) method and compare its handling of optional enum properties with the issue's suggested decoding behavior. Confirm the generated model accepts an unknown value for an optional enum without throwing while preserving normal decoding for known values.