Fix #73: setting values doesn't work just after initialization

Since options.defaultValues is only used on _create there is no point in
setting its value later through .option('defaultValues', ...).

Changing the range or bounds just after the initialization may yield to
the same kind of bug but this patch only takes care about setting the
values.
Merge pull request #74 from rosenfeld/master

Fix #73: setting values doesn't work just after initialization