title: mutate function is not updating
body: <p>I have a <code>tbl_df</code> with several columns.  I am running a simple mutate on it to update a column, but the column is not being updated.  </p>
  
  <pre><code>mutate(fltr, cat = "xxxxx")
  </code></pre>
  
  <p><code>cat</code> is a column that is either <code>empty</code> or <code>NA</code>.  Filter is a <code>tbl_df</code>.  What could be causing this?  I have tried to put text in the <code>cat</code> column so that it is not empty or NA in case that was causing the problem. That still did not work.</p>