ignoreWebXML flag use is opposite of what the name implies.

In order to create a war archive, you must call setIgnoreWebXML(false) which is counter intuitive.

/**

• flag set if finding the webxml is to be expected.
  */
  private boolean ignoreWebxml = true;

Seems like the name of this variable should be expectWebxml or the if should be changed below:
// If no webxml file is specified, it's an error.
if ( ignoreWebxml && deploymentDescriptor == null && !isInUpdateMode() )
{
throw new ArchiverException( "webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode)" );