perf: optimize HeadersList.get (nodejs#2420)

* perf: optimize HeadersList.get

* simple

lib/fetch/headers.js

@@ -208,15 +208,13 @@ class HeadersList {
 
   // https://fetch.spec.whatwg.org/#concept-header-list-get
   get (name) {
-    // 1. If list does not contain name, then return null.
-    if (!this.contains(name)) {
-      return null
-    }
+    const value = this[kHeadersMap].get(name.toLowerCase())
 
+    // 1. If list does not contain name, then return null.
     // 2. Return the values of all headers in list whose name
     //    is a byte-case-insensitive match for name,
     //    separated from each other by 0x2C 0x20, in order.
-    return this[kHeadersMap].get(name.toLowerCase())?.value ?? null
+    return value === undefined ? null : value.value
   }
 
   * [Symbol.iterator] () {