Add support for Elasticsearch.update

[sillly-milly]

From what I can tell, https://elasticsearch-py.readthedocs.io/en/7.x/api.html#elasticsearch.Elasticsearch.update is not supported by the mocking client (I think when I try to call the function on the mock then it's falling through to the super impl which tries to contact a server and of course there is no server so I get ConnectionRefusedError).

In my project I've decided to instead just use .index(id=...) to update documents, but I imagine the real client does some smarter, more efficient stuff via .update with more bells and whistles which I'm missing out on, so would be nice to support this!

By the way, loving the tool, it's made writing my tests so much easier!