Fix http.ClientRequest crashes if end() was called twice

Fixes #1417.
Fixes #1223.

lib/http.js

@@ -648,6 +648,9 @@ OutgoingMessage.prototype.addTrailers = function(headers) {
 
 
 OutgoingMessage.prototype.end = function(data, encoding) {
+  if (this.finished) {
+    return false;
+  }
   if (!this._header) {
     this._implicitHeader();
   }

test/simple/test-http-request-end-twice.js

@@ -0,0 +1,38 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var http = require('http');
+
+var server = http.Server(function(req, res) {
+  res.writeHead(200, {'Content-Type': 'text/plain'});
+  res.end("hello world\n");
+});
+server.listen(common.PORT, function() {
+  var req = http.get({port: common.PORT}, function(res) {
+    res.on('end', function() {
+      assert.ok(!req.end());
+      server.close();
+    });
+  });
+});
+