http: revert 06cfff9, fixes overridden options

This commit reverts commit 06cfff9 and adds a test that failed before
the revert. Options is in fact overridden in the code below the
util._extend() call (specifically options.port and options.host) to
allow for a one-argument net.createConnection(options) call.

lib/_http_client.js

@@ -21,6 +21,8 @@ function ClientRequest(options, cb) {
 
   if (typeof options === 'string') {
     options = url.parse(options);
+  } else {
+    options = util._extend({}, options);
   }
 
   var agent = options.agent;

test/parallel/test-http-dont-override-options.js

@@ -0,0 +1,43 @@
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+var requests = 0;
+
+http.createServer(function(req, res) {
+  res.writeHead(200);
+  res.end('ok');
+
+  requests++;
+}).listen(common.PORT).unref();
+
+var agent = new http.Agent();
+agent.defaultPort = common.PORT;
+
+// options marked as explicitly undefined for readability
+// in this test, they should STAY undefined as options should not
+// be mutable / modified
+var options = {
+  host: undefined,
+  hostname: 'localhost',
+  port: undefined,
+  defaultPort: undefined,
+  path: undefined,
+  method: undefined,
+  agent: agent
+};
+
+http.request(options, function(res) {
+  res.resume();
+}).end();
+
+process.on('exit', function() {
+  assert.equal(requests, 1);
+
+  assert.strictEqual(options.host, undefined);
+  assert.strictEqual(options.hostname, 'localhost');
+  assert.strictEqual(options.port, undefined);
+  assert.strictEqual(options.defaultPort, undefined);
+  assert.strictEqual(options.path, undefined);
+  assert.strictEqual(options.method, undefined);
+});