feat(client): add a Connection Pool

This adds a connection pool to the Client that is used by default. It
accepts any other NetworkConnector, and simply acts as a
NetworkConnector itself. Other Pools can exist by simply providing a
custom NetworkConnector. This Pool is only used by default if you also
use the default connector, which is `HttpConnector`. If you wish to use
the Pool with a custom connector, you'll need to create the Pool with
your custom connector, and then pass that pool to the
Client::with_connector.

This also adds a method to `NetworkStream`, `close`, which can be used
to know when the Stream should be put down, because a server requested
that the connection close instead of be kept alive.

Closes #363
Closes #41

src/server/mod.rs

@@ -36,13 +36,13 @@ pub use net::{Fresh, Streaming};
 use HttpError::HttpIoError;
 use {HttpResult};
 use buffer::BufReader;
-use header::{Headers, Connection, Expect};
-use header::ConnectionOption::{Close, KeepAlive};
+use header::{Headers, Expect};
+use http;
 use method::Method;
 use net::{NetworkListener, NetworkStream, HttpListener};
 use status::StatusCode;
 use uri::RequestUri;
-use version::HttpVersion::{Http10, Http11};
+use version::HttpVersion::Http11;
 
 use self::listener::ListenerPool;
 
@@ -206,11 +206,7 @@ where S: NetworkStream + Clone, H: Handler {
             }
         }
 
-        keep_alive = match (req.version, req.headers.get::<Connection>()) {
-            (Http10, Some(conn)) if !conn.contains(&KeepAlive) => false,
-            (Http11, Some(conn)) if conn.contains(&Close)  => false,
-            _ => true
-        };
+        keep_alive = http::should_keep_alive(req.version, &req.headers);
         let mut res = Response::new(&mut wrt);
         res.version = req.version;
         handler.handle(req, res);