feat(net): remove mut requirement for NetworkConnector.connect()

BREAKING CHANGE: Any custom Connectors will need to change to &self in
  the connect method. Any Connectors that needed the mutablity need to
  figure out a synchronization strategy.

  Request::with_connector() takes a &NetworkConnector instead of &mut.
  Any uses of with_connector will need to change to passing &C.

src/client/request.rs

@@ -47,7 +47,7 @@ impl Request<Fresh> {
     }
 
     /// Create a new client request with a specific underlying NetworkStream.
-    pub fn with_connector<C, S>(method: method::Method, url: Url, connector: &mut C)
+    pub fn with_connector<C, S>(method: method::Method, url: Url, connector: &C)
         -> ::Result<Request<Fresh>> where
         C: NetworkConnector<Stream=S>,
         S: Into<Box<NetworkStream + Send>> {