http: chunked: fix chunked transfer encoding output

Chunks in chunked transfer-encoding are supposed to be ended with \r\n. Without that, servers can be confused on how to parse the next chunk size, and are typically returning a 400 Bad Request. This can be seen in the Mozilla developer documentation¹, or more exhaustively in the document RFC² section 4.1.

This is in particular needed to signal that no other headers are exposed in the chunk.