mod_pastebin: Prevent header table form being mutated default_headers was mutated, probably by mod_http_errors and ended up with multiple content-type entries, which led to issues in browsers which would sometimes treat the paste as html

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>