enable Content-Type header charset

[yutackall]

Content-Type header , in the case described charset, JSON is not sent and query string sent.

code

let headers = {
  headers: {
    'Content-Type': 'application/json; charset=UTF-8'
  }
}
config.registerEndpoint('api', 'http://examle.com', headers);

[doktordirk]

merged #172