http: Replace "in" usage with "=== undefined"

Speeds up http benchmarks.

lib/http.js

@@ -352,7 +352,7 @@ IncomingMessage.prototype._addHeaderLine = function(field, value) {
   switch (field) {
     // Array headers:
     case 'set-cookie':
-      if (field in dest) {
+      if (dest[field] !== undefined) {
         dest[field].push(value);
       } else {
         dest[field] = [value];
@@ -372,7 +372,7 @@ IncomingMessage.prototype._addHeaderLine = function(field, value) {
     case 'proxy-authenticate':
     case 'sec-websocket-extensions':
     case 'sec-websocket-protocol':
-      if (field in dest) {
+      if (dest[field] !== undefined) {
         dest[field] += ', ' + value;
       } else {
         dest[field] = value;
@@ -383,14 +383,14 @@ IncomingMessage.prototype._addHeaderLine = function(field, value) {
     default:
       if (field.slice(0, 2) == 'x-') {
         // except for x-
-        if (field in dest) {
+        if (dest[field] !== undefined) {
           dest[field] += ', ' + value;
         } else {
           dest[field] = value;
         }
       } else {
         // drop duplicates
-        if (!(field in dest)) dest[field] = value;
+        if (dest[field] === undefined) dest[field] = value;
       }
       break;
   }
@@ -1010,7 +1010,7 @@ ServerResponse.prototype.writeHead = function(statusCode) {
       var field;
       for (var i = 0, len = obj.length; i < len; ++i) {
         field = obj[i][0];
-        if (field in headers) {
+        if (headers[field] !== undefined) {
           obj.push([field, headers[field]]);
         }
       }
@@ -1868,7 +1868,7 @@ function connectionListener(socket) {
       }
     });
 
-    if ('expect' in req.headers &&
+    if (req.headers.expect !== undefined &&
         (req.httpVersionMajor == 1 && req.httpVersionMinor == 1) &&
         continueExpression.test(req.headers['expect'])) {
       res._expect_continue = true;