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

tests/support/prelude.rs

@@ -32,7 +32,7 @@ pub use self::futures::{Future, IntoFuture, Sink, Stream};
 pub use super::future_ext::{FutureExt, Unwrap};
 
 // Re-export HTTP types
-pub use self::http::{HeaderMap, Method, Request, Response, StatusCode};
+pub use self::http::{HeaderMap, Method, Request, Response, StatusCode, Version};
 
 pub use self::bytes::{Buf, BufMut, Bytes, BytesMut, IntoBuf};