Date-picker's value is always changed from client after attach

[alexberazouski]

When the PolymerTemplate with date-picker inside is attached to the view I see in server-side logs that its value was changed twice:

date:  2018-06-11   old:  null
date:  2018-11-06   old:  2018-06-11

Seems that updating locale makes this weird change on client.
As a result it's impossible to use date-picker with binder and to do a dirty checking.