Add fetch default value for options hash

[amrani]

As stated in the documentation, a Field::Polymorphic should default its classes to an empty array.

• Default classes for a Field::Polymorphic
• Add spec coverage

[FanaHOVA]

.fetch raises a KeyError when the key is missing, it doesn't return false, so the second part of the conditional would have never ran in the previous code:

2.4.0 :001 > hash = {}
 => {} 
2.4.0 :002 > res = hash.fetch(:key1) || 'Something'
KeyError: key not found: :key1
	from (irb):2:in `fetch'
	from (irb):2
	from /Users/fana/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'
2.4.0 :003 > res
 => nil 

I'm wondering if users have been rescuing KeyError as a result and this will be a breaking change for them?

[amrani]

I think if someone were rescuing then it would be to return a default. In that case, the default is likely also an empty Array as it is consistent with other keys from options. Where do you think a user may have added the rescue? If they were overriding this method then this change would not affect them. 🤔 What do you think?

[FanaHOVA]

@amrani well right now if options doesn't have the classes key this will raise a KeyError, not sure about the whole architecture of the code so can't exactly tell you where that might have happened. Somebody with more codebase experience will probably chime in here :) But yea, rescues will be consistent with this new default probably.

[nickcharlton]

I'm not (off the top of my head) aware of anything which would catch this otherwise and I'd assume that that Dashboard would throw an exception.

[nickcharlton]

Thanks for catching this and adding the tests!