Fix optional parameters for commands

[spirifoxy]

When declaring commands with optional parameters, calling them omitting those parameters throws an exception.

Example:

 _dialogueRunner.AddCommandHandler<string, string>("Scene", Scene);
...

 private void Scene(string imageName, string alignment = "center") {
    ...

Calling this command as <<Scene village_intro>> will lead to this:

object System.Reflection.MethodBase.HandleTypeMissing(System.Reflection.ParameterInfo, System.RuntimeType): System.ArgumentException: Missing parameter does not have a default value. (Parameter 'parameters')
  <C++ Error>    System.ArgumentException
  <C++ Source>   :0 @ object System.Reflection.MethodBase.HandleTypeMissing(System.Reflection.ParameterInfo, System.RuntimeType)
  <Stack Trace>  :0 @ object System.Reflection.MethodBase.HandleTypeMissing(System.Reflection.ParameterInfo, System.RuntimeType)
                 :0 @ object System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(object, System.Reflection.BindingFlags, System.Reflection.Binder, System.Object[], System.Globalization.CultureInfo)
                 :0 @ object System.Delegate.DynamicInvokeImpl(System.Object[])

The fix is to send those default values directly instead of Type.Missing

[dogboydog]

Nice! I tested and it works great. Also still works if you override the default value. Thanks. Will release as 0.2.8