[MINOR] ensure that buffer_feed() and buffer_skip() set BF_*_PARTIAL

It's important that these functions set these flags themselves, otherwise
the callers will always have to do this, and there is no valid reason for
not doing it.
diff --git a/include/proto/buffers.h b/include/proto/buffers.h
index 432146c..ab47b4f 100644
--- a/include/proto/buffers.h
+++ b/include/proto/buffers.h
@@ -349,6 +349,9 @@
 	buf->send_max -= len;
 	if (!buf->send_max && !buf->pipe)
 		buf->flags |= BF_OUT_EMPTY;
+
+	/* notify that some data was written to the SI from the buffer */
+	buf->flags |= BF_WRITE_PARTIAL;
 }
 
 /*
diff --git a/src/buffers.c b/src/buffers.c
index a21eb4f..dd5c003 100644
--- a/src/buffers.c
+++ b/src/buffers.c
@@ -119,6 +119,8 @@
 	if (buf->l >= buf->max_len)
 		buf->flags |= BF_FULL;
 
+	/* notify that some data was read from the SI into the buffer */
+	buf->flags |= BF_READ_PARTIAL;
 	return -1;
 }