[Swift3] Inheritance decoding with discriminator fails (2.3 branch)

Description

When the client tries to decode a response to an object that has inheritance, decoding fails because of a casting error. We get a mismatched type error because it cannot cast Decoded<File> to Decoded<Resource>

Swagger-codegen version

2.3.0 branch

Swagger declaration file content or url

This is the structure of the objects in the swagger definitions:

  Resource:
    type: object
    description: Common meta data for resources (files and directories)
    discriminator: resource_type
    properties:
      id:
        description: The unique identifier of this file
        type: string
        readOnly: true
      resource_type:
        type: string
        enum: [File, Directory]
        readOnly: true
        example: File
  File:
    type: object
    description: Meta data of a file. Use the url property to access the actual file
    allOf:
      - $ref: '#/definitions/Resource'
      - type: object
        properties: <snip>
  Directory:
    type: object
    description: Meta data of a directory
    allOf:
      - $ref: '#/definitions/Resource'
      - type: object
        properties: <snip>

This is the code in Models.swift that does the decoding:

 // Decoder for Resource
        Decoders.addDecoder(clazz: Resource.self) { (source: AnyObject, instance: AnyObject?) -> Decoded<Resource> in
            if let sourceDictionary = source as? [AnyHashable: Any] {
                // Check discriminator to support inheritance
                if let discriminator = sourceDictionary["resource_type"] as? String, instance == nil && discriminator != "Resource"{
                    return Decoders.decode(clazz: Resource.self, discriminator: discriminator, source: source)
                }
  //etc

And it fails in the decode function defined in Models.mustache line 70
https://github.com/swagger-api/swagger-codegen/blob/2.3.0/modules/swagger-codegen/src/main/resources/swift3/Models.mustache#L70

I'm not a very experienced with Swift so I tried multiple ways of casting the type, or somehow enforcing that the type T extends some other type, but none of it worked, in the end you simply cannot cast Decoded<File> to Decoded<Resource>

Start with the generated Models.swift decoder shown in the issue and the decode function in modules/swagger-codegen/src/main/resources/swift3/Models.mustache at line 70. Reproduce the failure using the Resource, File, and Directory inheritance definitions with the resource_type discriminator. Done means decoding the inherited response no longer fails when the result is handled as Decoded.