Don't set missing keys to null.

If key is missing from input it should not be set
to null in resulting object because such object
can't be but back into encoder.

fixes #9
It's not me who submitted this pull request. @muromec I'm sorry for responding for you.
To explain it in more detail:
a little bit upper you can find the following:

    var result = state['default'];

So, even if `present === false` result could be `!==null`.