title: different total number of columns in add_header_above from kable
body: <p>I am using kableExtra to format tables in rmarkdown. In one of my dataframes I am obtaining a strange result... </p>
  
  <pre><code>ncol(Selected)
  Selected%&gt;%
  kable("html",format.args = list(decimal.mark = ".", big.mark = ","),caption = "Variables description  and data cleaning and treatment summary") %&gt;%
  add_header_above(c(" " = 2, "Kolmogorov-Smirnov" = 2, "Kruskall-Wallis" = 2, "Wilcoxon  Test"= 2)) %&gt;%
  kable_styling()
  </code></pre>
  
  <p>The object Selected is a dataframe, with dimension (256;8)...</p>
  
  <p>I got this:</p>
  
  <pre><code>[1] 8
  Error in htmlTable_add_header_above(kable_input, header, bold, italic, : The new header row you provided has a different total number of columns with the original kable output.
  </code></pre>
  
  <p>What is wrong? By ncol, I confirmed the object Selected has 8 columns... 
  Thanks in advance.</p>