code: @SuppressWarnings("WeakerAccess")
      public DatabaseConnectionRequest<DatabaseUser, AuthenticationException> createUser(@NonNull String email, @NonNull String password, @NonNull String connection) {
          //noinspection ConstantConditions
          return createUser(email, password, null, connection);
      }
docs: Creates a user in a DB connection using <a href="https://auth0.com/docs/api/authentication#signup">'/dbconnections/signup' endpoint</a>
  Example usage:
  <pre>
  {@code
  client.createUser("{email}", "{password}", "{database connection name}")
  .start(new BaseCallback<DatabaseUser>() {
  {@literal}Override
  public void onSuccess(DatabaseUser payload) { }
  
  {@literal}@Override
  public void onFailure(AuthenticationException error) { }
  });
  }
  </pre>
  
  @param email      of the user and must be non null
  @param password   of the user and must be non null
  @param connection of the database to create the user on
  @return a request to start