Always output document end before directive (YAML 1.2 compatibility)

In YAML 1.1, the document end marker `...` is optional even if the next document starts with a directive:
https://github.com/yaml/pyyaml/blob/master/tests/data/spec-07-09.canonical
```
%YAML 1.1
---
!!str "foo"
%YAML 1.1
---
!!str "bar"
%YAML 1.1
---
!!str "baz"
```
It is only required if the scalar is "open ended", for example for plain scalars.

In YAML 1.2 the `...` marker is always required before a directive.

My suggestion would be to make the output 1.2 compatible. It will still be 1.1 compatible, so that shouldn't be a problem.

I believe this will also make it easier to fix #123 which was introduced with the last fixes regarding `open_ended`. I think I can make a fix for this soon after this issue is fixed.
Fixing #123 without this would be a bit more complicated.

If we do this, we also need to adjust PyYAML to behave the same.

Related issues/commits:
- #60
- #122
- 56400d9, 8ee83c0, 56f4b17

src/emitter.c

@@ -594,6 +594,7 @@ yaml_emitter_emit_document_start(yaml_emitter_t *emitter,
         {
             if (!yaml_emitter_write_indicator(emitter, "...", 1, 0, 0))
                 return 0;
+            emitter->open_ended = 0;
             if (!yaml_emitter_write_indent(emitter))
                 return 0;
         }
@@ -644,6 +645,7 @@ yaml_emitter_emit_document_start(yaml_emitter_t *emitter,
 
         emitter->state = YAML_EMIT_DOCUMENT_CONTENT_STATE;
 
+        emitter->open_ended = 0;
         return 1;
     }
 
@@ -691,6 +693,7 @@ yaml_emitter_emit_document_end(yaml_emitter_t *emitter,
         if (!event->data.document_end.implicit) {
             if (!yaml_emitter_write_indicator(emitter, "...", 1, 0, 0))
                 return 0;
+            emitter->open_ended = 0;
             if (!yaml_emitter_write_indent(emitter))
                 return 0;
         }
@@ -1796,7 +1799,6 @@ yaml_emitter_write_indicator(yaml_emitter_t *emitter,
 
     emitter->whitespace = is_whitespace;
     emitter->indention = (emitter->indention && is_indention);
-    emitter->open_ended = 0;
 
     return 1;
 }
@@ -1939,10 +1941,6 @@ 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;
 }