[R] ApiClient defaultHeaderParams are ignored

Description

Here is the constructor for ApiClient:

    initialize = function(basePath, configuration, defaultHeaders){
        ...
        if (!missing(defaultHeaders)) {
            self$defaultHeaders <- defaultHeaders
        }
       ...
    },

defaultHeaders are not referenced anywhere else in the R code.

Swagger-codegen version

swagger-codegen-cli-2.4.0-20180821.200234-311

Swagger declaration file content or url

Any file, e.g. petstore

Command line used for generation

java -jar swagger-codegen-cli-2.4.0-20180821.200234-311.jar generate --lang r -i swagger.yaml

Steps to reproduce

apic <- ApiClient$new("http://localhost:4200", NULL, c('Content-Type' = "application/json"))
PetApi$new(apic)$getPetById(123)

Observe the lack of any headers in the request.

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/6520

Suggest a fix/enhancement

Merge this with the user-specified headers before sending the request, e.g. in ApiClient$callApi

Start with the R ApiClient constructor and ApiClient$callApi mentioned in the issue, then run the PetApi reproduction using the supplied Content-Type header. Trace how defaultHeaders and user-specified headers reach the request. Done means the generated R client sends the constructor's default header when calling getPetById.