BUG/MEDIUM: streams: Don't use CF_EOI to decide if the request is complete.

BUG/MEDIUM: streams: Don't use CF_EOI to decide if the request is complete.

In si_cs_send(), don't check CF_EOI on the request channel to decide if the
request is complete and if we should save the buffer to eventually attempt
L7 retries. The flag may not be set yet, and it may too be set to early,
before we're done modifying the buffer. Instead, get the msg, and make sure
its state is HTTP_MSG_DONE.
That way we will store the request buffer when sending it even in H2.