http: improve compatibility of legacy API

In http1, legacy http.Client shares one connection with multiple requests.
But in http2, it uses concurrent connections.
With --use-http1, test/simple/test-http-legacy.js passes.
However, it fails without --use-http1 (use http2).

This improves compatibility of legacy http.Client API between http1 and http2.

Fixes nodejs#1530.

lib/http2.js

@@ -1457,6 +1457,7 @@ function Client(port, host) {
   port = port || 80;
   this.host = host;
   this.port = port;
+  this.agent = new Agent({ host: host, port: port, maxSockets: 1 });
 }
 util.inherits(Client, EventEmitter);
 Client.prototype.request = function(method, path, headers) {
@@ -1472,6 +1473,7 @@ Client.prototype.request = function(method, path, headers) {
   options.method = method;
   options.path = path;
   options.headers = headers;
+  options.agent = self.agent;
   var c = new ClientRequest(options);
   c.on('error', function(e) {
     self.emit('error', e);

test/simple/test-http-legacy.js

@@ -0,0 +1,101 @@
+// 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 url = require('url');
+
+function p(x) {
+  common.error(common.inspect(x));
+}
+
+var responses_sent = 0;
+var responses_recvd = 0;
+var body0 = '';
+var body1 = '';
+
+var server = http.createServer(function(req, res) {
+  if (responses_sent == 0) {
+    assert.equal('GET', req.method);
+    assert.equal('/hello', url.parse(req.url).pathname);
+
+    console.dir(req.headers);
+    assert.equal(true, 'accept' in req.headers);
+    assert.equal('*/*', req.headers['accept']);
+
+    assert.equal(true, 'foo' in req.headers);
+    assert.equal('bar', req.headers['foo']);
+  }
+
+  if (responses_sent == 1) {
+    assert.equal('POST', req.method);
+    assert.equal('/world', url.parse(req.url).pathname);
+    this.close();
+  }
+
+  req.on('end', function() {
+    res.writeHead(200, {'Content-Type': 'text/plain'});
+    res.write('The path was ' + url.parse(req.url).pathname);
+    res.end();
+    responses_sent += 1;
+  });
+
+  //assert.equal('127.0.0.1', res.connection.remoteAddress);
+});
+
+server.listen(common.PORT, function() {
+  var client = http.createClient(common.PORT);
+  var req = client.request('/hello', {'Accept': '*/*', 'Foo': 'bar'});
+  setTimeout(function() {
+    req.end();
+  }, 100);
+  req.on('response', function(res) {
+    assert.equal(200, res.statusCode);
+    responses_recvd += 1;
+    res.setEncoding('utf8');
+    res.on('data', function(chunk) { body0 += chunk; });
+    common.debug('Got /hello response');
+  });
+
+  setTimeout(function() {
+    var req = client.request('POST', '/world');
+    req.end();
+    req.on('response', function(res) {
+      assert.equal(200, res.statusCode);
+      responses_recvd += 1;
+      res.setEncoding('utf8');
+      res.on('data', function(chunk) { body1 += chunk; });
+      common.debug('Got /world response');
+    });
+  }, 1);
+});
+
+process.on('exit', function() {
+  common.debug('responses_recvd: ' + responses_recvd);
+  assert.equal(2, responses_recvd);
+
+  common.debug('responses_sent: ' + responses_sent);
+  assert.equal(2, responses_sent);
+
+  assert.equal('The path was /hello', body0);
+  assert.equal('The path was /world', body1);
+});