Fix content type for jsn requests

[brainrepo]

During message sent, the api response is

{
  status: 500,
  message: {
    code: 500,
    message: "Content type 'text/plain;charset=UTF-8' not supported",
    tid: 'c7b2e249eae16f19',
    sid: 'c7b2e249eae16f19'
  }
}

with this fix, the correct content type is passed in headers