fix: fix variable typo for paypro.

lib/PayPro.js

@@ -214,7 +214,7 @@ PayPro.prototype.sign = function(key) {
     var pki_data = this.get('pki_data'); // contains one or more x509 certs
     //var details = this.get('serialized_payment_details');
     var type = pki_type.split('+').toUpperCase();
-    var verifier = crypto.createSign('RSA-' + type);
+    var signature = crypto.createSign('RSA-' + type);
     var buf = this.serializeForSig();
     signature.update(buf);
     //var pki_data = require('fs').readFileSync(__dirname + '/../test/data/x509.pem');