Silently attempt to fetch non-existent hash value

[timfjord]

When the array! method attempts to read a non-existent hash value it raises the KeyError error because it uses Hash#fetch method under the hood without the second argument.
This case is not covered with tests(the Hash case is not covered at all) so I assume it is not the expected behavior.
I cannot find any arguments when this behavior might be useful.

This PR fixes this issue and makes sure an attempt to read a non-existent hash value returns nil

[dhh]

Seems like its better to raise on a missing attribute than to silently return nil? Seems like a way you'd get screwed on spelling errors?

[timfjord]

I'd consider that this piece of code

data = [{ id: 1, content: 'hello', meta: 'meta' }, { id: 2, content: 'world' }] 

json.array! data, :id, :content, :meta

is a shortcut for

data = [{ id: 1, content: 'hello', meta: 'meta' }, { id: 2, content: 'world' }] 

json.array! data do |hash|
  json.id hash[:id]
  json.content hash[:content]
  json.meta hash[:meta]
end

So I would expect them to behave similarly.

Of course we can say, that instead of Hash#[] we should have used Hash#fetch, plus, indeed, #fetch can help us with
spelling errors, so maybe we could handle that with an additional option?

json.array! data, :id, :content, :meta, fetch: true # assuming that `Hash#[]` is the default method

or

json.array! data, :id, :content, :meta, silent: true # assuming that `Hash#fetch` is the default method

[timfjord]

Hm, we could even handle that completely differently and introduce default option so it can be used for both hashes and objects

json.array! data, :id, :content, :meta, default: ''