Normalize HTTP request path. (#228)

The HTTP/2.0 specification requires that the path pseudo header is never
empty for requests unless the request uses the OPTIONS method.

This is currently not correctly enforced.

This patch provides a test and a fix.

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, Version};
+pub use self::http::{uri, HeaderMap, Method, Request, Response, StatusCode, Version};
 
 pub use self::bytes::{Buf, BufMut, Bytes, BytesMut, IntoBuf};