Pass ...Expr_ConstFetch a ...Node_Name if isDefaultValueConstant is true

<img src="http://media.giphy.com/media/11bnQmUcjLzy00/giphy.gif" align="right" />

Currently PHPParser_Node_Expr_ConstFetch is [being passed a string](https://github.com/Ocramius/CodeGenerationUtils/blob/0.1.0/src/CodeGenerationUtils/ReflectionBuilder/ClassBuilder.php#L208-L210) by [ReflectionParameter::getDefaultValueConstantName](http://php.net/manual/en/reflectionparameter.getdefaultvalueconstantname.php) when the constructor actually expects a PHPParser_Node_Name as the first argument.

I've added a test to cover the code, happy to change the way I've implemented the test, couldn't work out how to cleanly pass ReflectionClass a class that has a method with the right arguments.

The error this resolves:

> Argument 1 passed to PHPParser_Node_Expr_ConstFetch::__construct() must be an instance of PHPParser_Node_Name, string given, called in CodeGenerationUtils/ReflectionBuilder/ClassBuilder.php on line 209
Nope, will use `PHP_VERSION`.

Magic constants however don't seem to return true for `isDefaultValueConstant`.