[typescript-fetch] Add support for file (application/octet-stream) response types

Description

Currently, swagger attempts to process file (application/octet-stream) response content as JSON. In particular, swagger calls response.json(), which results in an error.

Swagger-codegen version

v2.4.0 - master.

Swagger declaration file content or url

return fetch(basePath + localVarFetchArgs.url, localVarFetchArgs.options).then((response) => {
  if (response.status >= 200 && response.status < 300) {
    return response.json();  // Throws "Uncaught (in promise) SyntaxError: Unexpected token % in JSON at position 0"
  } else {
    throw response;
  }
});

Command line used for generation

java -jar ./other_modules/swagger/swagger-codegen-cli.jar generate -i http://localhost:8011/swagger/v1/swagger.json -l typescript-fetch -o ./other_modules/swagger/output

Steps to reproduce

Run swagger-codegen against an API that produces application/octet-stream content.

Suggest a fix/enhancement

Modify the api.mustache file for typescript-fetch as need be (make reference to typescript-angular module where support for file response types has already been added), using the isResponseFile flag. When isResponseFile is true, return the response content as blob -> return response.blob();