When `to_s` was [deprecated][1] for `to_fs`, warnings were added when
explicit formats were passed to `to_s`. To determine whether a format
argument was passed, the method signature was changed from
`to_s(format = :default)` to `to_s(format = EMPTY_OBJECT)`. While this
enabled being able to warn on formats passed, it changed the behavior of
not passing any format for applications that customized the `:default`
format for one of these classes.

This commit restores the previous behavior for `:default` formats while
keeping the existing warnings. Additionally, it will continue to not
warn for applications using `to_s` without arguments that haven't set a
custom `:default` format. However, for applications that have set a
custom `:default` format, it will now warn that those classes should be
migrated to `to_fs`.

[1]: f9fbfe0a