createUser = username => {
  fetch("http://localhost:3000/api/v1/users", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      Accept: "application/json"
    },
    body: JSON.stringify({
    username
  })
 })
   .then(res => res.json())
   .then(user => {
    let newArr = [...this.state.users, user];
    this.setState({ users: newArr, currentUser: user });
  });
};