Only update the rbush item if the extent has changed

[fredj]

To be able to update a feature property while iterating. Example:

source.forEachFeature(function(feature) {
  feature.set('foo', 'bar');
});

The feature was lost during the migration to CommonJS style modules in #2867.

See also #2219

[ahocevar]

Are you sure that the extent change works while iterating? Previously we threw exceptions in this case.

[fredj]

If the extent is modified while iterating an exception will be thrown in remove.

The exception could be thrown in update (as before)

[fredj]

An exception is now thrown if the extent is modified while reading

[ahocevar]

I think this is good now. I just restarted Travis to see if she agrees. But if she does, please merge.

[fredj]

Thanks for the review