Python client code is wrong when an endpoint accept multiple MediaTypes

Description

My OpenApi 3.0 specification contains an endpoint for which the response media type can be application/octet-stream or, if the query cannot be completed, application/json.
The generated Python client code builds a query with a MediaType being the concatenation of both: application/octet-streamapplication/json

Swagger-codegen version

Codegen 3.0.0-rc1

Swagger declaration file content or url

Below is my (simplified) OpenApi specification:

openapi: 3.0.0

servers:
- url: http://localhost/myapi

info:
  version: "1.0"
  title: My API

security:
- basicAuth: []

paths:

  /download:
    get:
      operationId: download
      responses:
        200:
          description: OK
          content:
            application/octet-stream:
              schema:
                type: string
                format: binary
        500:
          description: Error
          content:
            application/json:
              schema:
                type: string

Command line used for generation

I call Swagger Codegen directly from my Java application:

	CodegenConfigurator configurator = new CodegenConfigurator();
	configurator.setInputSpec("spec.yml");
	configurator.setGeneratorName("python");
	configurator.setOutputDir("out");
	new DefaultGenerator().opts(configurator.toClientOptInput()).generate();

Python test file

Running this script

import unittest
import openapi_client

configuration = openapi_client.Configuration()
configuration.debug = True

# create an instance of the API class
api_instance = openapi_client.DefaultApi(openapi_client.ApiClient(configuration))

class TestModels(unittest.TestCase):
    def test_download(self):
        api_instance.download()

if __name__ == '__main__':
    unittest.main()

results in the following output:

2018-07-02 15:17:38,841 DEBUG Starting new HTTP connection (1): localhost
send: 'GET /myapi HTTP/1.1\r\nHost: localhost:10080\r\nAccept-Encoding: identity\r\nContent-Type: application/json\r\nAccept: application/octet-streamapplication/json\r\nAuthorization: Basic c2VtYWRtaW46c2VtYWRtaW4=\r\nUser-Agent: OpenAPI-Generator/1.0.0/python\r\n\r\n'
reply: 'HTTP/1.1 406 Not Acceptable\r\n'
...

Start with the generated Python client's request construction for the /download operation and compare it with the OpenAPI response content entries for application/octet-stream and application/json. Reproduce the issue with the provided spec and test script. Done means the generated request sends valid separate media types and no longer concatenates them into application/octet-streamapplication/json.