i-bem, i-bem-dom: `findElem` should update elem's cache

[qfox]

Can findElem method rewrite elem's internal cache if called with the same parameters?

[aristov]

// cc @dfilatov

[veged]

Yes, it can do this in case of simple params. Feel free to make PR.

[qfox]

Related issue #598

[qfox]

@veged izi.