title: Non standard evaluation of dplyr summarise_ leads to different results
body: <p>I want call <code>summarise_</code> of <code>dplyr</code> package in my function, here is what I tried but I get different for <code>mean</code> and <code>median</code> function, what's wrong with my approach?</p>
  
  <pre><code>library(dplyr)
  df &lt;- data.frame(A=c(1,2,3))
  getMean &lt;- function(df, col) {
    col &lt;- as.symbol(col)
    df %&gt;%
      summarise_(Mean = ~mean(col))
  }
  
  getMedian &lt;- function(df, col) {
    col &lt;- as.symbol(col)
    df %&gt;%
      summarise_(Median = ~median(col))
  }
  
  getMean(df, 'A')
     Mean
  1    2
  
  getMedian(df, 'A')
  Error: object 'A' not found 
  </code></pre>