Raise disabled during boot inside fallback

[KaanOzkan]

Noticed this code path being hit with locale == false in our application after disabling any i18n calls during boot phase. It makes sense to raise Disabled here as well and let the user know about their mistake.

[radar]

Hey, thanks for the PR. Can you show me an example of how to make it break first?

[KaanOzkan]

I came across this break from a Rails method call.

class Dummy < ApplicationController
end

> Dummy.helpers
NoMethodError: undefined method `to_sym' for false:FalseClass
Did you mean?  to_s
from /.bundle/vendor/gems/i18n-1.6.0/lib/i18n/locale/fallbacks.rb:69:in `[]'

#helpers ends up calling i18n.fallback https://github.com/rails/rails/blob/master/actionview/lib/action_view/lookup_context.rb#L50 .

If it's called before initializing where locale is set to false we get the error. Actual fix will have to be in client code but figured we can give a better error message through raising Disabled.

[radar]

Seems like a good idea to me :) Can you please look into adding a test for this change too?

[KaanOzkan]

Let me know if you actually prefer this one to be in its own test class. I wasn't sure about adding it to this one.

[radar]

Seems fine :)