title: "object 'day' not found r". But 'day' is a column name
body: <p>I'm studying the main functions of <code>dplyr</code> package. When I type <code>flights</code> I have it:</p>
  
  <pre><code>year month   day dep_time sched_dep_time dep_delay
     &lt;int&gt; &lt;int&gt; &lt;int&gt;    &lt;int&gt;          &lt;int&gt;     &lt;dbl&gt;
  1   2013     1     1      517            515         2
  2   2013     1     1      533            529         4
  3   2013     1     1      542            540         2
  4   2013     1     1      544            545        -1
  5   2013     1     1      554            600        -6
  6   2013     1     1      554            558        -4
  7   2013     1     1      555            600        -5
  8   2013     1     1      557            600        -3
  9   2013     1     1      557            600        -3
  10  2013     1     1      558            600        -2
  </code></pre>
  
  <p>we can see <code>day</code> is a column name. When I type: </p>
  
  <pre><code>jan1 &lt;- filter (flights, month == 1, day==1)
  </code></pre>
  
  <p>I get the error message </p>
  
  <blockquote>
    <p>Error in match.arg(method) : object 'day' not found</p>
  </blockquote>
  
  <p>But it is a column name. Could you help me?</p>