title: all.equal on object with NULL names causes 'Error: not compatible with STRSXP' -- bug or expected?
body: <p>In the example below, when names are set to <code>NULL</code>, <code>all.equal</code> throws <code>'Error: not compatible with STRSXP'</code><br>
  However, if names are set to <code>NA</code> (or some other value), <code>all.equal</code> works as normal.<br>
  <strong>Is this expected behavior or a bug?</strong></p>
  
  <pre><code>## SAMPLE DATA
  set.seed(1)
  x &lt;- data.frame(LETTERS[1:3], rnorm(3))
  names(x) &lt;- NULL
  
  x
  #   NA        NA
  # 1  A -0.626454
  # 2  B  0.183643
  # 3  C -0.835629
  
  all.equal(x, x)
  # Error: not compatible with STRSXP
  
  # add names back in, even 'NA'
  names(x) &lt;- c(NA, NA)
  all.equal(x, x)
  # [1] TRUE
  </code></pre>