Client should validate request URI. (#181)

This patch adds checks for the request URI and rejects invalid URIs. In
the case of forwarding an HTTP 1.1 request with a path, an "http" pseudo
header is added to satisfy the HTTP/2.0 spec.

Closes #179

src/proto/streams/send.rs

@@ -74,8 +74,6 @@ impl Send {
             }
         }
 
-
-
         let end_stream = frame.is_end_stream();
 
         // Update the state

src/proto/streams/streams.rs

@@ -494,7 +494,8 @@ where
             }
 
             // Convert the message
-            let headers = client::Peer::convert_send_message(stream_id, request, end_of_stream);
+            let headers = client::Peer::convert_send_message(
+                stream_id, request, end_of_stream)?;
 
             let mut stream = me.store.insert(stream.id, stream);