[typescript-fetch] Bug: For an API method which "produces" : [ "text/plain" ], response.json() is called in the generated client code

Description

For a swagger operation which produces a plain text response body (MIME type text/plain), defined as "produces" : [ "text/plain" ], the generated client code calls the json-function of the response object, which tries to decode the response body as JSON.

                    if (response.status >= 200 && response.status < 300) {
                        return response.json();
                    } else {
                        throw response;
                    }

This fails at runtime with an SyntaxError: Unexpected token x in JSON at position 0

Swagger-codegen version

2.4.2

Swagger declaration file content or url

{
  "swagger" : "2.0",
  "info" : {
    "version" : "1.0",
    "title" : "API"
  },
  "host" : "localhost:8888",
  "tags" : [ {
    "name" : "Test"
  } ],
  "schemes" : [ "http" ],
  "paths" : {
    "/test" : {
      "get" : {
        "tags" : [ "test" ],
        "summary" : "plain text test",
        "operationId" : "getPlainText",
        "produces" : [ "text/plain" ],
        "parameters" : [ {
          "name" : "key",
          "in" : "query",
          "required" : true,
          "type" : "string"
        } ],
        "responses" : {
          "200" : {
            "description" : "successful operation",
            "schema" : {
              "type" : "string"
            }
          }
        }
      }
    }
  }
}

Command line used for generation

swagger-codegen-maven-plugin:2.4.2:generate (default)
using <language>typescript-fetch</language>

Steps to reproduce

Generate typescript-fetch code from the swagger declaration file (see above)

Suggest a fix/enhancement

The response callback must either respect the MIME types specified in the produces section of the operation, or alternatively the Content-Type header of the response can be evaluated at runtime.

Start by running swagger-codegen-maven-plugin:2.4.2:generate with language typescript-fetch against the supplied Swagger declaration and inspect the generated getPlainText method. Trace how its successful response is selected and verify completion by generating a client that handles the text/plain string response without the reported JSON decode failure.