[Swift4] Accept and Content-type headers should be available as specified in swagger specs

Description

Accept headers should be validated with the allowed accept header in the produces section of the YAML specs

Content-type header should be set as the one from YAML

so that the request going out is well constructed per the specs set in the YAML file.

This is happening in the ObjC code generated from the same snapshot but not in the Swift code

Swagger-codegen version

2.3.0 latest snapshot 18th November

Swagger declaration file content or url

      consumes:
        - application/json;charset=UTF-8
      produces:
        - application/json

Command line used for generation

java -jar swagger-codegen-2.3.0.jar generate -c SwaggerConfig.json -i MySwagger.yaml -l swift4 -o /tmp/20-Nov-17

Steps to reproduce

set the above consumes and produces in the YAML file and generate Swift Swagger code or ObjC Swagger code then the request generated is never validated for the headers being set appropriately in the API.swift file

Related issues/PRs

Suggest a fix/enhancement

1, make public methods available that vend out this information as specified in the YAML eg.
at its simplest. This way the consuming client for this pod can use this in even when it is constructing the Datarequest in it's own way.

public func valueFor(headerName: String) -> String? {
let headers = 
["Content-Type": "application/json;charset=UTF-8",  //from the consumes section
  "Accept": "application/json" //from produces section
]
return headers[headerName]
}

Start with the generated Swift API.swift from the documented swagger-codegen command and compare its Accept and Content-Type handling with the generated ObjC client. Trace how the YAML consumes and produces values reach the request, then verify that Swift requests use those values and expose the requested header information.