perf: improve isomorphic decode performance

lib/fetch/util.js

@@ -871,13 +871,8 @@ function isomorphicDecode (input) {
   // 1. To isomorphic decode a byte sequence input, return a string whose code point
   //    length is equal to input’s length and whose code points have the same values
   //    as the values of input’s bytes, in the same order.
-  let output = ''
 
-  for (let i = 0; i < input.length; i++) {
-    output += String.fromCharCode(input[i])
-  }
-
-  return output
+  return input.reduce((previous, current) => previous + String.fromCharCode(current), '')
 }
 
 /**