Fix for wrong image name for internal functions

When an internal function is used in a namespaced class the FunctionPostfix image name is set with the namespace. It is not completely wrong because you can create the same function name in the namespace (ie below), but it is rare.

Internal function in namespace:

```
namespace nspace;

function var_dump($var) {
  echo 'This is my custom var_dump';
}
```

To make be distinct, you can use the `\`, but it is not usual for internal function in namespaced apps.

It can happen to user functions too, but is hard do determine with the parser, once the function can be defined in different files.

This pull request just cover the case where internal functions are used in namespaced apps.