enable more use cases by returning the xml object in callback

lib/signed-xml.js

@@ -820,7 +820,7 @@ SignedXml.prototype.computeSignature = function(xml, opts, callback) {
         signatureDoc.insertBefore(self.createSignature(prefix), signedInfoNode.nextSibling)
         self.signatureXml = signatureDoc.toString()
         self.signedXml = doc.toString()
-        callback()
+        callback(null, self)
       }
     })
   }