http: emit abort event from ClientRequest

ClientRequest will now emit an `abort` event to detect when `abort` is
called.

doc/api/http.markdown

@@ -859,6 +859,12 @@ Emitted when the server sends a '100 Continue' HTTP response, usually because
 the request contained 'Expect: 100-continue'. This is an instruction that
 the client should send the request body.
 
+### Event: 'abort'
+
+`function () { }`
+
+Emitted when the request has been aborted by the client.
+
 ### request.flush()
 
 Flush the request headers.

lib/_http_client.js

@@ -166,6 +166,12 @@ ClientRequest.prototype._implicitHeader = function() {
 };
 
 ClientRequest.prototype.abort = function() {
+  var self = this;
+  if (this.aborted === undefined) {
+    process.nextTick(function() {
+      self.emit('abort');
+    });
+  }
   // Mark as aborting so we can avoid sending queued request data
   // This is used as a truthy flag elsewhere. The use of Date.now is for
   // debugging purposes only.

test/parallel/test-http-client-abort-event.js

@@ -0,0 +1,28 @@
+var assert = require('assert');
+var http = require('http');
+var common = require('../common');
+var server = http.createServer(function(req, res) {
+  res.end();
+});
+var count = 0;
+server.listen(common.PORT, function() {
+  var req = http.request({
+    port: common.PORT
+  }, function() {
+    assert(false, 'should not receive data');
+  });
+
+  req.on('abort', function() {
+    // should only be emitted once
+    count++;
+    server.close();
+  });
+
+  req.end();
+  req.abort();
+  req.abort();
+});
+
+process.on('exit', function() {
+  assert.equal(count, 1);
+})