Abstract everything but the response from hyper

Because we're using unsized types not for requesting, there's not a
satisfactory way of doing this without boxing the request...

Once unsized stuff lands in rust 1.2/1.3(???) then this should be
implemented with Rc's instead of Box's.

For the time being I'm not sure what else to do.

servo/servo#6727

tests/unit/net/http_loader.rs

@@ -2,24 +2,23 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use net::http_loader::{load, LoadError, HttpRequester};
+use net::http_loader::{load, LoadError, HttpRequester, HttpRequest};
 use url::Url;
 use std::sync::{Arc, Mutex};
 use ipc_channel::ipc;
 use net_traits::LoadData;
 use net::hsts::HSTSList;
-use hyper::client::{Request, Response};
-use hyper::net::Fresh;
+use hyper::client::Response;
 use hyper::method::Method;
 
 struct MockHttpRequester;
 
 impl HttpRequester for MockHttpRequester {
-    fn build(&self, url: Url, _: Method) -> Result<Request<Fresh>, LoadError> {
+    fn build(&self, url: Url, _: Method) -> Result<Box<HttpRequest>, LoadError> {
         Err(LoadError::Connection(url.clone(), "shouldn't connect".to_string()))
     }
 
-    fn send(&self, _: Request<Fresh>) -> Result<Response, LoadError> {
+    fn send(&self, _: Box<HttpRequest>) -> Result<Response, LoadError> {
         Err(LoadError::Connection(Url::parse("http://example.com").unwrap(), "shouldn't connect".to_string()))
     }
 }