title: Using dplyr summarise_at with column index
body: <p>I noticed that when supplying column indices to <code>dplyr::summarize_at</code> the column to be summarized is determined excluding the grouping column(s). I wonder if that is how it's supposed to be since by this design, using the correct column index depends on whether the summarising column(s) are positioned before or after the grouping columns.</p>
  
  <p>Here's an example:</p>
  
  <pre><code>library(dplyr)
  data("mtcars")
  
  # grouping column after summarise columns
  mtcars %&gt;% group_by(gear) %&gt;% summarise_at(3:4, mean)
  ## A tibble: 3 x 3
  #   gear     disp       hp
  #  &lt;dbl&gt;    &lt;dbl&gt;    &lt;dbl&gt;
  #1     3 326.3000 176.1333
  #2     4 123.0167  89.5000
  #3     5 202.4800 195.6000
  
  # grouping columns before summarise columns
  mtcars %&gt;% group_by(cyl) %&gt;% summarise_at(3:4, mean)
  ## A tibble: 3 x 3
  #    cyl        hp     drat
  #  &lt;dbl&gt;     &lt;dbl&gt;    &lt;dbl&gt;
  #1     4  82.63636 4.070909
  #2     6 122.28571 3.585714
  #3     8 209.21429 3.229286
  
  # no grouping columns
  mtcars %&gt;% summarise_at(3:4, mean)
  #      disp       hp
  #1 230.7219 146.6875
  
  # actual third &amp; fourth columns
  names(mtcars)[3:4]
  #[1] "disp" "hp"  
  
  packageVersion("dplyr")
  #[1] ‘0.7.2’
  </code></pre>
  
  <p>Notice how the summarised columns change depending on grouping and position of the grouping column.</p>
  
  <p>Is this the same on other platforms? Is it a bug or a feature?</p>