perf: use insertion sort algorithm (nodejs#2706)

lib/fetch/headers.js

@@ -454,11 +454,26 @@ class Headers {
 
     // 2. Let names be the result of convert header names to a sorted-lowercase
     //    set with all the names of the headers in list.
-    const names = [...this[kHeadersList]].sort((a, b) => a[0] < b[0] ? -1 : 1)
+    const names = [...this[kHeadersList]]
+    const namesLength = names.length
+    if (namesLength <= 16) {
+      // Note: Use insertion sort for small arrays.
+      for (let i = 1, value, j = 0; i < namesLength; ++i) {
+        value = names[i]
+        for (j = i - 1; j >= 0; --j) {
+          if (names[j][0] <= value[0]) break
+          names[j + 1] = names[j]
+        }
+        names[j + 1] = value
+      }
+    } else {
+      names.sort((a, b) => a[0] < b[0] ? -1 : 1)
+    }
+
     const cookies = this[kHeadersList].cookies
 
     // 3. For each name of names:
-    for (let i = 0; i < names.length; ++i) {
+    for (let i = 0; i < namesLength; ++i) {
       const [name, value] = names[i]
       // 1. If name is `set-cookie`, then:
       if (name === 'set-cookie') {