BUG/MINOR: stream_sock: don't remove BF_EXPECT_MORE and BF_SEND_DONTWAIT on partial writes

The flags are one-shot but should be maintained over all send() operations
as long as send_max is not flushed. The flags were incidentely cleared once
a complete send() was performed, regardless of the fact that the send()
might have been on the first half of a buffer before a wrapping. The result
is that on wrapping data (eg: which happens often with chunked encoding),
many incomplete segments are transmitted instead of being aggregated.

The fix consists in only flushing the flags only once send_max is empty,
which was the expected behaviour.

This fix should be backported to 1.4 though it is not critical, just sub-optimal.
diff --git a/src/stream_sock.c b/src/stream_sock.c
index 29b7fcb..8c9f765 100644
--- a/src/stream_sock.c
+++ b/src/stream_sock.c
@@ -633,10 +633,6 @@
 				send_flag &= ~MSG_MORE;
 
 			ret = send(si->fd, b->w, max, send_flag);
-
-			/* Always clear both flags once everything has been sent */
-			if (ret == max)
-				b->flags &= ~(BF_EXPECT_MORE | BF_SEND_DONTWAIT);
 		} else {
 			int skerr;
 			socklen_t lskerr = sizeof(skerr);
@@ -668,6 +664,8 @@
 
 			b->send_max -= ret;
 			if (!b->send_max) {
+				/* Always clear both flags once everything has been sent, they're one-shot */
+				b->flags &= ~(BF_EXPECT_MORE | BF_SEND_DONTWAIT);
 				if (likely(!b->pipe))
 					b->flags |= BF_OUT_EMPTY;
 				break;