fix: match net module headers & http.IncomingMessage headers (#17517)

* fix: match net module headers & http.IncomingMessage headers

* update net doc for cleanliness

* address feedback from review

* Update spec/api-net-spec.js

Co-Authored-By: codebytere <codebytere@github.com>

* add special cookie case

lib/browser/api/net.js

@@ -16,6 +16,29 @@ Object.setPrototypeOf(URLRequest.prototype, EventEmitter.prototype)
 
 const kSupportedProtocols = new Set(['http:', 'https:'])
 
+// set of headers that Node.js discards duplicates for
+// see https://nodejs.org/api/http.html#http_message_headers
+const discardableDuplicateHeaders = new Set([
+  'content-type',
+  'content-length',
+  'user-agent',
+  'referer',
+  'host',
+  'authorization',
+  'proxy-authorization',
+  'if-modified-since',
+  'if-unmodified-since',
+  'from',
+  'location',
+  'max-forwards',
+  'retry-after',
+  'etag',
+  'last-modified',
+  'server',
+  'age',
+  'expires'
+])
+
 class IncomingMessage extends Readable {
   constructor (urlRequest) {
     super()
@@ -41,7 +64,23 @@ class IncomingMessage extends Readable {
   }
 
   get headers () {
-    return this.urlRequest.rawResponseHeaders
+    const filteredHeaders = {}
+    const rawHeaders = this.urlRequest.rawResponseHeaders
+    Object.keys(rawHeaders).forEach(header => {
+      if (header in filteredHeaders && discardableDuplicateHeaders.has(header)) {
+        // do nothing with discardable duplicate headers
+      } else {
+        if (header === 'set-cookie') {
+          // keep set-cookie as an array per Node.js rules
+          // see https://nodejs.org/api/http.html#http_message_headers
+          filteredHeaders[header] = rawHeaders[header]
+        } else {
+          // for non-cookie headers, the values are joined together with ', '
+          filteredHeaders[header] = rawHeaders[header].join(', ')
+        }
+      }
+    })
+    return filteredHeaders
   }
 
   get httpVersion () {