title: Does Enumerable's group_by preserve the Enumerable's order?
body: <p>Does <a href="http://ruby-doc.org/core-2.1.2/Enumerable.html#method-i-group_by" rel="noreferrer"><code>Enumerable#group_by</code></a> preserve the original order within each value? When I get this:</p>
  
  <pre><code>[1, 2, 3, 4, 5].group_by{|i| i % 2}
  # =&gt; {1=&gt;[1, 3, 5], 0=&gt;[2, 4]}
  </code></pre>
  
  <p>is it guaranteed that, for example, the array <code>[1, 3, 5]</code> contains the elements in this order and not, for example <code>[3, 1, 5]</code>?</p>
  
  <p>Is there any description regarding this point?</p>
  
  <p>I am not mentioning the order between the keys <code>1</code> and <code>0</code>. That is a different issue.</p>