Update the labels on separation

[ku]

When mergeRangeLabelsIfSame is set to true, two labels are combined once their values got close.
They got separated again if the values got away enough, but the text of a label stays the same with old one which is shown when it was combined.

Following gif shows the problem described above.

This patch solves the problem by updating the labels on separating combined labels.

[codecov-io]

Codecov Report

Merging #502 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #502   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         932    934    +2     
=====================================
+ Hits          932    934    +2

Impacted Files	Coverage Δ
src/rzslider.js	100% <100%> (ø)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2d7fb91...275ce3f. Read the comment docs.

[ValentinH]

Sorry for the delay, I haven't found the time to merge it for now (I want to merge and release it in the same time that's why). I'll try to do it soon. 😄