[Swift4] Unable to Download Files

[RedDragonJ]

Description

After generating our Swagger client, we were not able to download files. We found similar issue
that was fixed in the past but it doesn't seems to works for us.

Swagger-codegen version

2.3.1

Command line used for generation

swagger-codegen generate -i swagger.json -l swift4 -o SwaggerClient/

Related issues/PRs

This is similar change but did not fix our issue.
#6469

Suggest a fix/enhancement

We identified our code went through AlamofireDecodableRequestBuilder instead of AlamofireRequestBuilder.
We will add a new pull request that adds the file download code to
this class.

[RedDragonJ]

Created a pull request
#8700