fix(http2): received Body::size_hint() now return 0 if implicitly empty (#2715)

An HTTP/2 stream may include a set of headers, and a flag signalling
END-STREAM, even if a `content-length` isn't included. hyper wouldn't
notice, and so the `Body` would report a size-hint of `0..MAX`. hyper
now notices that the stream is ended, and couldn't possibly include any
bytes for the body, and thus will give a size-hint of `0` exactly.

src/body/body.rs

@@ -204,9 +204,14 @@ impl Body {
     #[cfg(all(feature = "http2", any(feature = "client", feature = "server")))]
     pub(crate) fn h2(
         recv: h2::RecvStream,
-        content_length: DecodedLength,
+        mut content_length: DecodedLength,
         ping: ping::Recorder,
     ) -> Self {
+        // If the stream is already EOS, then the "unknown length" is clearly
+        // actually ZERO.
+        if !content_length.is_exact() && recv.is_end_stream() {
+            content_length = DecodedLength::ZERO;
+        }
         let body = Body::new(Kind::H2 {
             ping,
             content_length,