[BUG] [javascript][ApiClient] Fix request "responseType"

Created by: edouardmercier

In the master branch, the JavaScript ApiClient uses the responseTypewith value string when the returnTypeparameter is set to String, which, on its turns attempts to the superagent XMLHttpRequest responseType with value string (which is not supported and generates a warning on the browser developer tools console with message "The provided value 'string' is not a valid enum value of type XMLHttpRequestResponseType.", see https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/responseType) instead of text.

Could you please change line 522 of the file src/main/resources/Javascript/ApiClient.mustache with

request.responseType('text');

?