title: Map.get cannot retrieve value with string keys
body: <p>I'm having a strange issue with <code>Map</code>s. Here's a small example to reproduce the behaviour:</p>
  
  <pre><code>defmodule Document do
  
    @expiration_presets_to_timeshift %{
      "immediately": [seconds: 1],
      "1-minute": [minutes: 1],
      "1-hour": [hours: 1],
      "1-day": [days: 1],
      "1-week": [weeks: 1],
      "1-month": [months: 1],
      "1-year": [years: 1],
      "never": [years: 9999],
    }
  
    def expiration_presets, do: @expiration_presets_to_timeshift
  end
  </code></pre>
  
  <p>If we then try to retrieve information from the map:</p>
  
  <pre><code>iex(1)&gt; import Document
  Document
  iex(2)&gt; Document.expiration_presets
  %{"1-day": [days: 1], "1-hour": [hours: 1], "1-minute": [minutes: 1],
  "1-month": [months: 1], "1-week": [weeks: 1], "1-year": [years: 1],
  immediately: [seconds: 1], never: [years: 9999]}
  iex(3)&gt; Document.expiration_presets["1-hour"]
  nil
  iex(4)&gt; Map.get(Document.expiration_presets, "1-hour")
  nil
  </code></pre>
  
  <p>I notice two strange behaviors:</p>
  
  <ol>
  <li>The "immediately" and "never" keys have become atoms. I never asked for this. Why ?</li>
  <li>I can't seem to retrieve the value using the key strings (<code>Map.get/2</code> and <code>Map.[key]</code> both return <code>nil</code>). Why is that and how can I retrieve the value out of the map ?</li>
  </ol>