don't clear internal _props hash when saving storables

clearing _props during save() leads to re-instantiation of mapped objects and thus to
hard-to-debug behaviour:

    var book = Book.get(1);
    var author = new Author({book: book});
    author.save();

without this fix author.book would not be equal to book anymore since it's a new instance.
modified behaviour of storables reg. undefined values during construction time

if a storable is instantiated with undefined properties sqlstore now returns null
for them on access. before this fix a storable would return `undefined` for properties
that haven't been defined in the properties hash during instantiation, but return
`null` once the storable is fetched from db or cache. this is consistent with the
behaviour of storables loaded freshly from db.