feat(client): remove generic parameter for Connector

Closes #379

BREAKING CHANGE: For people using the default HttpConnector and Client,
    everything should continue to just work. If the Client has been
    used with a generic parameter, it should be removed.

    However, there were some breaking changes to the internals of
    NetworkConnectors. Specifically, they no longer return a
    NetworkStream, but instead a Into<Box<NetworkStream + Send>>. All
    implementations of NetworkStream should continue to just work,
    however.

    Possible breakages could come from the stricter usage of Send
    throughout the Client API.

src/lib.rs

@@ -190,6 +190,7 @@ mod mimewrapper {
 
 #[allow(unconditional_recursion)]
 fn _assert_send<T: Send>() {
+    _assert_send::<Client>();
     _assert_send::<client::Request<net::Fresh>>();
     _assert_send::<client::Response>();
 }