stream_send(...) should return Err(Error::Done) if no bytes are sent
stream_send(...) should return Err(Error::Done) if no bytes are sent

Motivation:

The documention of stream_send(...) states that Err(Error::Done) is returned if no bytes were send. This is not always the case and Ok(0) might be returned.

Modifications:

Return Err(Error::Done) if tx_cap is 0.

Result:

Return value matches the documentation.

Fixes #928.
cloudflare/quiche,cloudflare/quiche,cloudflare/quiche