Revert removing of open_ended after top level plain scalar

See also issue #60

Revert "This code is not needed and breaks tests"

This reverts commit 56400d976a.

src/emitter.c

@@ -1946,6 +1946,10 @@ yaml_emitter_write_plain_scalar(yaml_emitter_t *emitter,
 
     emitter->whitespace = 0;
     emitter->indention = 0;
+    if (emitter->root_context)
+    {
+        emitter->open_ended = 1;
+    }
 
     return 1;
 }