ActiveStorage::Filename was missing quotes when encoded,
generating invalid json like this -
```
JSON.generate(foo: ActiveStorage::Filename.new("bar.pdf") # => '{"foo":bar.pdf}'
```
Delete to_json and rely on the implementation from ActiveSupport::ToJsonWithActiveSupportEncoder
Active Storage is an engine which means its models, jobs and controllers
are autoloaded by Rails rather than Ruby. Unfortunately this means it's
subject to the same gotchas as applications, including this one:
http://guides.rubyonrails.org/v5.1.4/autoloading_and_reloading_constants.html#when-constants-aren-t-missed-qualified-references
On Ruby < 2.5, constants nested under classes can't be autoloaded by
Rails if a top level constant already exists with the same name.
To avoid clashing with constants defined in users' applications or gems,
we can use `require_dependency` to ensure that the nested constants are
loaded before they're used.
