gfe-relnote: Fixes HPACK encoding of SpdyHeaderBlocks when compression is disabled. No functional change in production. For HTTP/2, compression is always enabled in production. Shinkansen metadata uses a different HpackEncoder API that is not affected by this CL. PiperOrigin-RevId: 285208542 Change-Id: I8c755500bb4c1004d264c7663bc4d6308a0b06ce