[Refactor] new implementation for utils.combine

lib/utils.js

@@ -212,7 +212,28 @@ var isBuffer = function isBuffer(obj) {
 };
 
 var combine = function combine(a, b) {
-    return [].concat(a, b);
+    // we always use both of these, so, let's calculate them now
+    var firstIsArray = Array.isArray(a);
+    var secondIsArray = Array.isArray(b);
+
+    // mutate `a` to append `b` and then return it
+    if (firstIsArray) {
+        if (secondIsArray) {
+            a.push.apply(a, b);
+        } else {
+            a.push(b);
+        }
+        return a;
+    }
+
+    // mutate `b` to prepend `a` and then return it
+    if (secondIsArray) {
+        b.unshift(a);
+        return b;
+    }
+
+    // neither are arrays, so, create a new array with both
+    return [a, b];
 };
 
 module.exports = {