Fix a regression when the Cache class is subclassed

See:
#219 (comment)

flask_caching/__init__.py

@@ -223,7 +223,7 @@ def _set_cache(self, app: Flask, config) -> None:
         import_me = config["CACHE_TYPE"]
         if "." not in import_me:
             plain_name_used = True
-            import_me = type(self).__module__ + ".backends." + import_me
+            import_me = "flask_caching.backends." + import_me
         else:
             plain_name_used = False
 

tests/test_basic_app.py

@@ -13,6 +13,9 @@
     HAS_NOT_REDIS = True
 
 
+class CustomCache(Cache):
+    pass
+
 class CustomSimpleCache(SimpleCache):
     pass
 
@@ -123,3 +126,12 @@ def test_app_custom_cache_backend(app):
 
     with app.app_context():
         assert isinstance(cache.cache, CustomSimpleCache)
+
+
+def test_subclassed_cache_class(app):
+    # just invoking it here proofs that everything worked when subclassing
+    # otherwise an werkzeug.utils.ImportStringError exception will be raised
+    # because flask-caching can't find the backend
+
+    # testing for "not raises" looked more hacky like this..
+    CustomCache(app)