title: RCurl postForm issues with fieldnames with special characters
body: <p>How do I enclose a field name to postForm with RCurl when the form has fields like those below?</p>
  
  <pre><code>&lt;input id="form:checkEstrato" type="checkbox" name="form:checkEstrato" checked="checked" /&gt;
  
  &lt;input id="form:checkArea" type="checkbox" name="form:checkArea" checked="checked" /&gt;
  </code></pre>
  
  <p>if I try something like</p>
  
  <pre><code>if(url.exists(url)) 
    results &lt;- postForm(url,
             form:evento="35",
             form:area = "10")
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>&gt; if(url.exists(url)) 
  +   results &lt;- postForm(url,
  +            form:evento="35",
  Error: unexpected '=' in:
  "  results &lt;- postForm(url,
             form:evento="
  &gt;            form:area = "10")
  Error: unexpected ')' in "           form:area = "10")"
  </code></pre>