[SWIFT4] POST payload being sent as Base64Encoded data (under "jsonData") instead of normal JSON

Description

For some reason, the generated SWIFT4 code is sending our POST payload as a single-key dictionary like:
[ "jsonData": "base64EncodedString"]

instead of a [String:Any] payload
our server (GCP Endpoints) does not like it and I don't understand why it's happening

Swagger-codegen version

<modelVersion>4.0.0</modelVersion>
<groupId>io.swagger</groupId>
<artifactId>swagger-codegen-project</artifactId>
<packaging>pom</packaging>
<name>swagger-codegen-project</name>
<version>2.4.0-SNAPSHOT</version>

Command line used for generation

java -jar /Users/dgaedcke/dev/swagger-codegen/swagger-codegen-cli.jar generate -c apigen_config.json -l swift4 -i /Users/dgaedcke/dev/tsSwagger/tstone1_0_1openapi.json -o /Users/dgaedcke/Downloads/TsApiBackend

apigen_config is:
{
"responseAs": "PromiseKit",
"projectName": "TsApi"
}

Steps to reproduce

I'm mostly asking a question here ... is this something we're doing wrong or some config-parameter we're missing here?

Start by reproducing the swift4 generation command with apigen_config.json and the referenced OpenAPI input, then inspect the generated client in the output directory for the POST request body. Compare the generated jsonData payload with the reported [String:Any] expectation; done means the cause and required generator change are identified and verified against the reproduction.