fix: implement Headers#set correctly (#2432)

* fix: implement `Headers#set` correctly

* fix: HeadersList

* test: add

* test: fix

lib/fetch/headers.js

@@ -190,7 +190,7 @@ class HeadersList {
     //    the first such header to value and remove the
     //    others.
     // 2. Otherwise, append header (name, value) to list.
-    return this[kHeadersMap].set(lowercaseName, { name, value })
+    this[kHeadersMap].set(lowercaseName, { name, value })
   }
 
   // https://fetch.spec.whatwg.org/#concept-header-list-delete
@@ -401,7 +401,7 @@ class Headers {
     // 7. Set (name, value) in this’s header list.
     // 8. If this’s guard is "request-no-cors", then remove
     //    privileged no-CORS request headers from this
-    return this[kHeadersList].set(name, value)
+    this[kHeadersList].set(name, value)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-getsetcookie

test/fetch/headers.js

@@ -250,7 +250,7 @@ tap.test('Headers has', t => {
 })
 
 tap.test('Headers set', t => {
-  t.plan(4)
+  t.plan(5)
 
   t.test('sets valid header entry to instance', t => {
     t.plan(2)
@@ -294,6 +294,16 @@ tap.test('Headers set', t => {
     t.throws(() => headers.set('undici'), 'throws on missing value')
     t.throws(() => headers.set('invalid @ header ? name', 'valid value'), 'throws on invalid name')
   })
+
+  // https://github.com/nodejs/undici/issues/2431
+  t.test('`Headers#set` returns undefined', t => {
+    t.plan(2)
+    const headers = new Headers()
+
+    t.same(headers.set('a', 'b'), undefined)
+
+    t.notOk(headers.set('c', 'd') instanceof Map)
+  })
 })
 
 tap.test('Headers forEach', t => {