http: add response.headersSent property

Boolean property getter. True if headers was sent, false otherwise.

doc/api/http.markdown

@@ -398,6 +398,10 @@ or
 
     response.setHeader("Set-Cookie", ["type=ninja", "language=javascript"]);
 
+### response.headersSent
+
+Boolean (read-only). True if headers were sent, false otherwise.
+
 ### response.sendDate
 
 When true, the Date header will be automatically generated and sent in 

lib/http.js

@@ -699,6 +699,12 @@ OutgoingMessage.prototype._renderHeaders = function() {
 };
 
 
+Object.defineProperty(OutgoingMessage.prototype, 'headersSent', {
+  configurable: true,
+  enumerable: true,
+  get: function () { return !!this._header; }
+});
+
 
 OutgoingMessage.prototype.write = function(chunk, encoding) {
   if (!this._header) {

test/simple/test-http-header-read.js

@@ -30,7 +30,9 @@ var s = http.createServer(function(req, res) {
   var contentType = 'Content-Type';
   var plain = 'text/plain';
   res.setHeader(contentType, plain);
+  assert.ok(!res.headersSent);
   res.writeHead(200);
+  assert.ok(res.headersSent);
   res.end('hello world\n');
   // This checks that after the headers have been sent, getHeader works
   // and does not throw an exception (Issue 752)