[ruby-core:96512] [Ruby master Bug#16457] Invisible keys on hash when defining hash with Hash.new({})

Issue #16457 has been reported by Farhad (Farhad Eyvazli).

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Bug #16457: Invisible keys on hash when defining hash with Hash.new({})
https://bugs.ruby-lang.org/issues/16457

* Author: Farhad (Farhad Eyvazli)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.6.3
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
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When using ``` Hash.new()``` to initialize a hash, we all know for undefined it will return specific value which sent as a parameter to the ```Hash.new``
But when doing something like that keys get invisible

```ruby
my_hash: Hash.new({})

my_hasy[:my_key] #=> {}
my_hash[:my_key].merge!(value: '')

my_hash.keys #=> []
my_hash.fetch(:my_key) #=> KeyError: key not found: :my_key

my_hash.dig(:my_key) #=> {:value=>""}
my_hash[:my_key] #=> {:value=>""}
```

Maybe it's normal behavior because, for each missing key, it initialize new empty has and merge it to that. But I'm not sure it can cause a memory leak or not when removing the main hash (:my_hash)
 



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