http: IncomingBody stream end isnt always an error

[pchickey]

https://github.com/yoshuawuyts/wstd/blob/pch/global_reactor/src/http/response.rs#L124

Its not an error path that the connection has closed unless the bytes read out of the stream doesn't match the content-length (if given).

See also #7 we should provide a read_to_end method on AsyncRead (or as AsyncReadExt like tokio does).