[swift5] Swift compiler warning, cast always fails

Description

In all our generated swift5-clients we get the following method that causes a warning:

    public convenience init(response: HTTPURLResponse, body: T?) {
        let rawHeader = response.allHeaderFields
        var header = [String:String]()
        for case let (key, value) as (String, String) in rawHeader {
            header[key] = value
        }
        self.init(statusCode: response.statusCode, header: header, body: body)
    }

Swift Compiler Warning:
swift5-client/SwaggerClient/Classes/Swaggers/Models.swift:31:35:
Cast from 'Dictionary<AnyHashable, Any>.Element'
(aka '(key: AnyHashable, value: Any)')
to unrelated type '(String, String)' always fails

Swagger-codegen version

macOS, homebrew, swagger-codegen version 3.0.18

Swagger declaration file content or url

openapi: 3.0.0
info:
  title: Thingies App
  version: "1"
  description: |+
    ...

paths:
  /thingies/load_all:
    post:
      responses:
        200:
          description: Load all thingies
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Thingies"
servers:
  - url: https://thingies.test/api
components:
  schemas:
    Thingies:
      type: object
      properties:
        property_one:
          type: string

Command line used for generation

swagger-codegen generate -l swift5 -i "${YAML_FILE}" -o "${SWIFT_PATH}"

Suggest a fix/enhancement

The line that causes the warning is this. Not familiar enough with this construct to fix it:

        for case let (key, value) as (String, String) in rawHeader {

Locate the swift5 client template that emits the HTTPURLResponse initializer and inspect how the raw headers are iterated. Run the provided swagger-codegen command with the sample OpenAPI document, then verify the generated Swift client no longer produces the reported cast warning.