Element.plain() behaves differently compared to Retangular.stripRestangular #1475

Please see the Plunkr https://plnkr.co/edit/ZeugLGt13u0uqYqlRU94

elem.plain() returns the plain version of the original model object even if it is called on a modified version of the model. I don't know if this is the intended behavior. If yes, a mention of this difference in the document would be appreciated.

Thanks,
Shyam