refactor: replace deprecated substr() with slice() (#1099)

Replace all instances of String.prototype.substr() with String.prototype.slice()
to use the standard, non-deprecated method. This maintains compatibility with
Node.js >= 0.8.0 while following modern JavaScript best practices.

All replacements maintain identical functionality as slice() and substr() behave
the same for these use cases.

Co-authored-by: bjohansebas <[email protected]>

index.js

@@ -593,7 +593,7 @@ function getcookie(req, name, secrets) {
     raw = cookies[name];
 
     if (raw) {
-      if (raw.substr(0, 2) === 's:') {
+      if (raw.slice(0, 2) === 's:') {
         val = unsigncookie(raw.slice(2), secrets);
 
         if (val === false) {
@@ -620,7 +620,7 @@ function getcookie(req, name, secrets) {
     raw = req.cookies[name];
 
     if (raw) {
-      if (raw.substr(0, 2) === 's:') {
+      if (raw.slice(0, 2) === 's:') {
         val = unsigncookie(raw.slice(2), secrets);
 
         if (val) {
@@ -696,7 +696,7 @@ function issecure(req, trustProxy) {
   var header = req.headers['x-forwarded-proto'] || '';
   var index = header.indexOf(',');
   var proto = index !== -1
-    ? header.substr(0, index).toLowerCase().trim()
+    ? header.slice(0, index).toLowerCase().trim()
     : header.toLowerCase().trim()
 
   return proto === 'https';

test/session.js

@@ -1299,7 +1299,7 @@ describe('session()', function(){
         var store = new session.MemoryStore()
         var server = createServer({ resave: false, store: store }, function (req, res) {
           if (req.method === 'PUT') {
-            req.session.token = req.url.substr(1)
+            req.session.token = req.url.slice(1)
           }
           res.end('token=' + (req.session.token || ''))
         })