title: openxlsx: read.xlsx throws an error if the sheet name contains the "&" character
body: <ol>
  <li><p>Create an <code>.xlsx</code> file with three sheets named: "Test 1", "S&amp;P500 TR" and "SP500 TR". Put some random content in each sheet and save it as "Book1.xlsx".</p></li>
  <li><p>Run:</p>
  
  <pre><code>&gt; a &lt;- getSheetNames("Book1.xlsx")
  &gt; a
  [1] "Test 1"    "S&amp;P500 TR" "SP500 TR"
  </code></pre></li>
  <li><p>Now try:</p>
  
  <pre><code>&gt; read.xlsx("Book1.xlsx", a[2])
  Error in read.xlsx.default("Book1.xlsx", a[2]) : 
    Cannot find sheet named "S&amp;P500 TR"
  </code></pre></li>
  </ol>