Run.set_tags won't accept string, irrespective of documentation

In at least versions 1.22.0 and 1.28.0, the [documentation for tagging runs](https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.run(class)?view=azure-ml-py#set-tags-tags-) states that one can pass a string, like `run.set_tags("foo")`. However, since `RunHistoryFacade.set_tags`calls `WorkspaceClient._sanitize_tags`, this will fail, as the latter calls the method `items` on its argument.