Fix slider format

[davidbrochart]

Fixes #3102

[davidbrochart]

There is another bug when using continuous_update=True, the front-end sometimes receives value updates with options={} although we should have an options.updated_view equal to this, and so there is an infinite value update even though this view originated the change.

[davidbrochart]

This was caused by to returning a string, and so did get, see the documentation:
"Note that if the .to() method returns a Number, noUiSlider's .get() will also return Numbers."
Should be OK now.

[martinRenou]

Tried locally, worked properly on my side