/// Returns the value of the attribute with the given key.
/// This is particulary important when a response contains special
/// attributes that are dependent on the action that has been sent.
/// An example of this is the response to the GetVarAction.
/// It contains the value of the channel variable as an attribute
/// stored under the key of the variable name.
/// Example:
///
/// GetVarAction action = new GetVarAction();
/// action.setChannel("SIP/1310-22c3");
/// action.setVariable("ALERT_INFO");
/// ManagerResponse response = connection.SendAction(action);
/// String alertInfo = response.getAttribute("ALERT_INFO");
///
/// As all attributes are internally stored in lower case the key is
/// automatically converted to lower case before lookup.
///
///