Modify TimeWithZone#as_json to return 3DP of sub-second accuracy by default, since it's allowed by the spec and is very useful.
ruby constant syntax is not supported as routing `:controller` option.

The current implementation only works correctly if you supply the `:controller`
with directory notation (eg. `:controller => 'admin/posts'`).

The ruby constant notation (eg. `:controller => 'Admin::Posts`) leads to unexpected problems with `url_for`.

This patch prints a warning for every non supported `:controller` option. I also added documentation how
to work with namespaced controllers. The warning links to that documentation in the rails guide.
Merge pull request #9039 from senny/warn_on_controller_option_with_ruby_constant_syntax

ruby constant syntax is not supported as routing `:controller` option.
Update actionpack/CHANGELOG.md

[ci skip]
Merge pull request #9128 from jamesotron/iso8601-sub-second-accuracy

Modify TimeWithZone#as_json to return 3DP of sub-second accuracy.