XML : Set escaped stringValue on NSXMLNode

I use NSXMLDocument to parse a HTML document. When trying to set a specific attribute of an NSXMLElement (which is of type NSXMLNode) like this

  attr?.setStringValue("a", resolvingEntities: false)    

And then fetching the resulting XML string from the document with

  document?.XMLString    

Will result in the following output

  <element attr="&amp;#61;">    

instead of what I really intended:

  <element attr="&#61;">    

Is there some way I can directly specify the string exactly how I want it to look like in the resulting XMLString? The reason for this special requirement is that I want to "obfuscate" mailto: links with html entities.