title: Metamethod when accessing a key as mutable
body: <p><code>__index</code> is called when accessing as immutable :</p>
  
  <pre><code>local foo = bar["foo"];
  </code></pre>
  
  <p><code>__newindex</code> is called when access as mutable an index that doesn't exist :</p>
  
  <pre><code>local bar = { }
  bar["foo"] = 123 -- calls __newindex
  bar["foo"] = 456 -- does NOT call __newindex
  </code></pre>
  
  <p>Is there a metamethod that can be called when accessing a key as mutable evey time, i.e not only if the key doesn't exist yet?</p>
  
  <p>I would like to create a behavior so that when a users sets a key in a table, it calls a native method instead, regardless if the key already exists or not.</p>