formatNum doesn't round correctly

[jscheffner]

• I've looked at the documentation to make sure the behavior is documented and expected
• I'm sure this is a Leaflet code issue, not an issue with my own code nor with the framework I'm using (Cordova, Ionic, Angular, React…)
• I've searched through the issues to make sure it's not yet reported

Steps to reproduce
run L.Util.formatNum(13.714814633131029, 15)

Expected behavior
Since the number already has 15 decimal points I would expect it to stay the same.

Current behavior
It returns 13.714814633131027

Environment

• Leaflet version: 1.6.0
• Browser (with version): Chrome but verified it in Karma
• OS/Platform (with version): OSX Mojave

Additional context

Minimal example reproducing the issue

https://next.plnkr.co/edit/p8zxeYBVQiz0v0FH

• this example is as simple as possible
• this example does not rely on any third party code

[IvanSanchez]

Errors from the 15th decimal on are within the expected precision of IEEE 64-bit floating point numbers. Won't fix.

See also #6668 and related.