http: remove the circular dependency

Between `ClientRequest` and `Agent`. The circular require was doing
weird things at load time, like making the `globalAgent` property
be `undefined` from within the context of the "_http_client"
module.

Removing the circular dependency completely fixes this.

This commit effectively removes the undocumented `Agent#request()`
and `Agent#get()` functions.

lib/_http_agent.js

@@ -20,10 +20,8 @@
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 var net = require('net');
-var url = require('url');
 var util = require('util');
 var EventEmitter = require('events').EventEmitter;
-var ClientRequest = require('_http_client').ClientRequest;
 var debug = util.debuglog('http');
 
 // New Agent code.
@@ -267,44 +265,4 @@ Agent.prototype.destroy = function() {
   });
 };
 
-Agent.prototype.request = function(options, cb) {
-  if (util.isString(options)) {
-    options = url.parse(options);
-  }
-  // don't try to do dns lookups of foo.com:8080, just foo.com
-  if (options.hostname) {
-    options.host = options.hostname;
-  }
-
-  if (options && options.path && / /.test(options.path)) {
-    // The actual regex is more like /[^A-Za-z0-9\-._~!$&'()*+,;=/:@]/
-    // with an additional rule for ignoring percentage-escaped characters
-    // but that's a) hard to capture in a regular expression that performs
-    // well, and b) possibly too restrictive for real-world usage. That's
-    // why it only scans for spaces because those are guaranteed to create
-    // an invalid request.
-    throw new TypeError('Request path contains unescaped characters.');
-  } else if (options.protocol && options.protocol !== this.protocol) {
-    throw new Error('Protocol:' + options.protocol + ' not supported.');
-  }
-
-  options = util._extend({
-    agent: this,
-    keepAlive: this.keepAlive
-  }, options);
-
-  // if it's false, then make a new one, just like this one.
-  if (options.agent === false)
-    options.agent = new this.constructor();
-
-  debug('agent.request', options);
-  return new ClientRequest(options, cb);
-};
-
-Agent.prototype.get = function(options, cb) {
-  var req = this.request(options, cb);
-  req.end();
-  return req;
-};
-
 exports.globalAgent = new Agent();