fix cache bug

[BenZhang]

Hi,
I ran into a very strange situation.

json.array! [1,2,3] do |i|
  json.cache! 'test' do
    json.name 'cache'
  end
  json.id i
end

The code above will print

[
  {"name":"cache","id":1},
  {"name":"cache","id":3},
  {"name":"cache","id":3}
]

If we move json.id on the top, it will work. The reason for that is when use cache! in the beginning of the block it will retrieve the same cached object from previous iteration and overwrite it. I think it is the same reason for this issue #330

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @pixeltrix (or someone else) soon.

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[BenZhang]

After further investigation, this will only happen when using memory_store for the cache. It is actually caused by the behaviour of Rails cache. Please see below example

Rails.cache.write('user', {id: 1})
a = Rails.cache.read('user')
#=> {:id=>1}
a[:name] = "Hello"
Rails.cache.read('user')
#=> {:id=>1, :name => "Hello"}

[ryanb]

I ran into this today. It would be great to get it fixed! Can someone address this PR?

[EvTi]

One more bug, i think

json.cache_collection! [1,2,3] do |category|
  next
end

result

{}
{}
{}