Preserve deferred loading of netCDF aux coords.

Fix for #1287.

(Replaces #1290.)
On reflection - we shouldn't be changing the repr in a bug fix release. Similarly, we shouldn't be changing the API of `LazyArray`. I'd be more comfortable putting this onto master. If you really want in 1.7.x, then we could make the dtype a keyword arg, but that doesn't make a lot of sense to me.