Add options[:app_env] to force the environment from the client initializer.

What?
====
What the tittle says.

Test
====
See ['PR # 3893`](https://github.com/TheClymb/theclymb/pull/3893)
Should we verify that APP_ENV is defined before this check to avoid an unintialized constant error? Something like:

```ruby
if options[:app_env] == :production || (defined?(APP_ENV) && APP_ENV == :production)
  # ...
```

It'd be great if we can avoid the APP_ENV constant altogether, since it's only used here.