BUG/MEDIUM: proxy: Don't forget the SF_HTX flag when upgrading TCP=>H1+HTX.

In stream_end_backend(), if we're upgrading from TCP to H1/HTX, as we don't
destroy the stream, we have to add the SF_HTX flag on the stream, or bad
things will happen.
This was broken when attempting to fix github issue #196.

This should be backported to 2.0.

src/proxy.c

@@ -1471,6 +1471,7 @@ int stream_set_backend(struct stream *s, struct proxy *be)
 					s->flags |= SF_IGNORE;
 					return 0;
 				}
+				s->flags |= SF_HTX;
 			}
 		}