title: dplyr: Arrange not behaving as expected after group_by and summarize
body: <p>I must be missing something with how <code>group_by</code> levels in <code>dplyr</code> get peeled off. In the example below, I group by 2 columns, summarize values into a single variable, then sort by that new variable: </p>
  
  <pre><code>mtcars %&gt;% group_by( cyl, gear ) %&gt;% 
    summarize( hp_range = max(hp) - min(mpg)) %&gt;% 
    arrange( desc(hp_range) )
  
  # Source: local data frame [8 x 3]
  # Groups: cyl [3]
  #
  #    cyl  gear  hp_range
  #  (dbl) (dbl) (dbl)
  #1     4     4  87.6
  #2     4     5  87.0
  #3     4     3  75.5
  #4     6     5 155.3
  #5     6     4 105.2
  #6     6     3  91.9
  #7     8     5 320.0
  #8     8     3 234.6
  </code></pre>
  
  <p>Obviously this is not sorted by <code>hp_range</code> as intended. What am I missing?</p>
  
  <p>EDIT: The example works as expected without the call to <code>desc</code> in arrange. Still unclear why?</p>