`Array::push` and others explicitly apply default formatting, rather than implicitly.

Found via rust-lang/cargo#15084 where a `&str` was `push`ed into an `Array` and then `sort_by_key`ed.  This made it so the formatting applied by `push` was not relevant.  If we delay until we render, then we'll know how to handle this.

The default formatting was added in #88.  I think after that we added the tracking of "this is in a default state" and wouldn't need this anymore.