fix ApiClient.setAccessToken

[christian-frei]

currently the method is of no use:

  /**
   * Helper method to set access token for the first OAuth2 authentication.
   *
   * @param accessToken Access token
   */
  public void setAccessToken(String accessToken) {
    throw new RuntimeException("No OAuth2 authentication configured!");
  }

I propose to make it a convenience method as follows:

  public void setAccessToken(String accessToken) {
    this.addDefaultHeader("Authorization", "Bearer " + accessToken);
  }