fix update api url and add tests

`ElasticSearch.update()` calls use the wrong url (it should end in '_update', see http://www.elasticsearch.org/guide/reference/api/update.html).

This patch breaks the existing update test, however since it's using the wrong url I'm not sure why it's passing (I'm not entirely sure what it's trying to test). So I replaced the test with the calls on the es update api page.

This change also changes the params for `update()`. I couldn't work out how to specify the script `lang` parameter to the es update api so I've left it out.