Rework appcontext: don't use a defaultdict, use Headers for headers

flask_rest_api/etag.py

@@ -21,7 +21,7 @@ def _is_etag_enabled():
 
 def _get_etag_ctx():
     """Get ETag section of AppContext"""
-    return get_appcontext()['etag']
+    return get_appcontext().setdefault('etag', {})
 
 
 class EtagMixin:

flask_rest_api/utils.py

@@ -1,6 +1,5 @@
 """Utils"""
 
-from collections import defaultdict
 from collections.abc import Mapping
 
 from werkzeug.datastructures import Headers
@@ -26,10 +25,12 @@ def deepupdate(original, update):
 def get_appcontext():
     """Return extension section from top of appcontext stack"""
 
-    # http://flask.pocoo.org/docs/0.12/extensiondev/#the-extension-code
+    # http://flask.pocoo.org/docs/latest/extensiondev/#the-extension-code
     ctx = _app_ctx_stack.top
     if not hasattr(ctx, 'flask_rest_api'):
-        ctx.flask_rest_api = defaultdict(dict)
+        ctx.flask_rest_api = {
+            'headers': Headers(),
+        }
     return ctx.flask_rest_api