Pass ConstFetch a Node_Name if isDefaultValueConstant is true.

Previously PHPParser_Node_Expr_ConstFetch was being passed a string
by ReflectionParameter::getDefaultValueConstantName when the class
actually expects a PHPParser_Node_Name as the constructors first argument.
Merge pull request #1 from sjparkinson/hotfix/parameterIsDefaultValueConstant

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