Fix issue with saving doSmartPreheat snippet in settings (#1)

self.settings.plugins.smartpreheat won't exist without at least one setting defined. Present a dummy setting for now.

octoprint_smartpreheat/__init__.py

@@ -57,7 +57,7 @@ def on_settings_initialized(self):
             self._settings.saveScript("gcode", "snippets/doSmartPreheat", u'' + script.replace("\r\n", "\n").replace("\r", "\n"))
 
     def get_settings_defaults(self):
-        return dict()
+        return dict(dummy=False)
 
     def get_template_configs(self):
         return [