title: Split with map! doesn't work
body: <p>Why map! doens't work with a splited string ?</p>
  
  <pre><code>str = "abcd"  
  str.split('').map! { |x| x + "!" }
  str # =&gt; "abcd"
  </code></pre>
  
  <p>I expected this would return <code>["a!","b!","c!","d!"]</code></p>