[Swift4] Dictionary generated, not [AnyHashable: String]

Description

A schema of type object and additionalProperties type: string generates a Swift Dictionary, I would expect a [AnyHashable: String]:

public struct Test: Codable {


    public var test: Dictionary
    public init(test: Dictionary) { 
        self.test = test
    }

}

Swagger-codegen version

3.0.3

Swagger declaration file content or url

openapi: 3.0.0
paths:
  /:
    get:
      parameters:
      responses:
        '200':
          content:
components:
  schemas:
    Test:
      required:
        - test
      type: object
      properties: 
        test:
          type: object
          additionalProperties:
            type: string

Command line used for generation

swagger-codegen generate -i test.yaml -l swift4

Reproduce the issue with the provided OpenAPI YAML and swagger-codegen generate -i test.yaml -l swift4 command. Start by tracing the Swift4 generator templates or type-mapping entry point that produces public var test: Dictionary; done means the generated property and initializer use the expected [AnyHashable: String] type.