Fix: fill `reserved_words` attribute instead the originally undefined attribute `reserved`

[rsyarif]

When parsing, reserved_words attribute never gets filled, instead a new originally undefined attr reserved is created and filled instead.

This is my simple fix to resolve https://github.com/s/preprocessor/issues/55

This may create issues for users who have used the attr reserved though. Not sure what the best approach whether to keep reserved filled or not.