Convert arrow func for backcompat

Author: LoneRifle

lib/signed-xml.js

@@ -810,15 +810,16 @@ SignedXml.prototype.computeSignature = function(xml, opts, callback) {
     this.signatureXml = signatureDoc.toString()
     this.signedXml = doc.toString()
   } else {
+    var self = this
     //Asynchronous flow
-    this.calculateSignatureValue(doc, (err, signature) => {
+    this.calculateSignatureValue(doc, function(err, signature) {
       if (err) {
         callback(err)
       } else {
-        this.signatureValue = signature
-        signatureDoc.insertBefore(this.createSignature(prefix), signedInfoNode.nextSibling)
-        this.signatureXml = signatureDoc.toString()
-        this.signedXml = doc.toString()
+        self.signatureValue = signature
+        signatureDoc.insertBefore(self.createSignature(prefix), signedInfoNode.nextSibling)
+        self.signatureXml = signatureDoc.toString()
+        self.signedXml = doc.toString()
         callback()
       }
     })