`Code.string_to_quoted/2` `static_atoms_encoder` is not called for quoted keys

[sabiwara]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.12.1 (compiled with Erlang/OTP 24) (reproduced on master)
• Operating system: Ubuntu 18.04

Current behavior

iex> Code.string_to_quoted(~S(["foo bar": :baz]), static_atoms_encoder: fn x, _ -> {:ok, x} end)
{:ok, ["foo bar": "baz"]}

The :"foo bar" atom actually gets created. If adding existing_atoms_only, it fails:

iex> Code.string_to_quoted(~S(["foo bar 2": :baz]), static_atoms_encoder: fn x, _ -> {:ok, x} end, existing_atoms_only: true)
** (ArgumentError) errors were found at the given arguments:

  * 1st argument: invalid UTF8 encoding

    :erlang.binary_to_existing_atom("foo bar 2", :utf8)

Expected behavior

The encoder should be called on all static atoms and prevent atom leaks:

iex> Code.string_to_quoted(~S(["foo bar": :baz]), static_atoms_encoder: fn x, _ -> {:ok, x} end)
{:ok, [{"foo bar", "baz"}]}

Note: I have a working fix, will open a PR.