fix(client): return error if Request has CONNECT method

The higher-level `Client` has never supported `CONNECT` requests,
but it used to send them, and then handle the responses incorrectly.
Now, it will return an error immediately instead of misbehaving.

src/client/mod.rs

@@ -138,9 +138,15 @@ where C: Connect,
             }
         }
 
+        if req.method() == &Method::Connect {
+            debug!("Client does not support CONNECT requests");
+            return FutureResponse(Box::new(future::err(::Error::Method)));
+        }
+
         let domain = match uri::scheme_and_authority(req.uri()) {
             Some(uri) => uri,
             None => {
+                debug!("request uri does not include scheme and authority");
                 return FutureResponse(Box::new(future::err(::Error::Io(
                     io::Error::new(
                         io::ErrorKind::InvalidInput,