[Swift4] Update Alamofire implementations to remove deprecated use of '!' when downcasting error responses

Description

I would like the AlamofireImplementations.swift to be updated, removing the deprecated use of '!' when downcasting error responses.

Swagger-codegen version

Code gen v 2.4.0

Swagger declaration file content or url

AlamofireImplementations.swift: Line 127

ErrorResponse.error(stringResponse.response?.statusCode ?? 500, stringResponse.data, stringResponse.result.error as Error!)

AlamofireImplementations.swift: Line 329

ErrorResponse.error(stringResponse.response?.statusCode ?? 500, stringResponse.data, stringResponse.result.error as Error!)

Command line used for generation

swagger-codegen generate -i $mysource -l swift4 -o /path/to/output -c my-config.json

Steps to reproduce

1. Install Swagger Code Gen v. 2.4.0
2. Generate client for Swift 4
3. Use generated client in project
4. Build project

Suggest a fix/enhancement

Since the error is optional, consider making the error optional in the ErrorResponse defined in Models.swift

public enum ErrorResponse: Error {
    case error(Int, Data?, Error?)
}

It's also recommended to update the dependency in the .podspec to Alamofire 4.7.1.

Start with the Swift 4 generation path and inspect the two reported locations in AlamofireImplementations.swift, lines 127 and 329, along with ErrorResponse in Models.swift. Regenerate a client with the shown command and confirm the generated code builds without the deprecated downcast usage; review the .podspec dependency reference as part of the result.