[query-params-new] Don't watch default model params obj

Subtle bug fix with query-params-new, whereby if you
specified queryParams on your ObjectController but
didn't provide you own model hook, the Ember-supplied
model hook, which returns the params object as the 
model in certain cases, was resulting in that params
object being observed, when definitely should not be,
so we make a copy of the POJO before returning it
as the model

Fixes #4155