[Proposal] System.put_env/2 clears env on getting nil as value

[Michal Śledź]

System.put_env/1 erases given key when a value is nil.

The proposal is to make System.put_env/2 symmetric with the System.put_env/1 and also erase a key on the nil value.

The spec would change from:

@spec put_env(binary(), binary()) :: :ok

to:

@spec put_env(binary(), binary() | nil) :: :ok

[José Valim]

Sounds good to me. PR welcome!

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[Michal Śledź]

I did the change and when updating tests I noticed there is already `delete_env` and now I am no longer sure about modyfing put_env. If we do it, we should probably deprecate delete_env? 

[Wojtek Mach]

While rather uncommon to delete things by setting to nil, I think parity with put_env/1 is worth it. I think it is fine to have put_env and delete_env. delete_env matches Application.delete_env, {Map,Keyword}.delete, etc, so I’d definitely keep it.

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