be explicit about sqlite memory uri

Author: davidism

flask_sqlalchemy/__init__.py

@@ -805,10 +805,13 @@ def init_app(self, app):
         of an application not initialized that way or connections will
         leak.
         """
-        if app.config.get('SQLALCHEMY_DATABASE_URI') is None:
-            warnings.warn(UserWarning('SQLALCHEMY_DATABASE_URI not set. Defaulting to sqlite.'))
+        if 'SQLALCHEMY_DATABASE_URI' not in app.config:
+            warnings.warn(
+                'SQLALCHEMY_DATABASE_URI not set. Defaulting to '
+                '"sqlite:///:memory:".'
+            )
 
-        app.config.setdefault('SQLALCHEMY_DATABASE_URI', 'sqlite://')
+        app.config.setdefault('SQLALCHEMY_DATABASE_URI', 'sqlite:///:memory:')
         app.config.setdefault('SQLALCHEMY_BINDS', None)
         app.config.setdefault('SQLALCHEMY_NATIVE_UNICODE', None)
         app.config.setdefault('SQLALCHEMY_ECHO', False)