rename params to data in post() to match patch() better

the implementation of `patch()` uses the name `data` for `json.loads(request.get_data()) or {}`, but the implementation of `post()` uses the name `params` for the same thing. This confused me while I was working on some experimental features; as "parameters" usually refers to URL parameters, not the body of a POST or PUT, and in fact the implementation of `_search()` uses the name `search_params` for data pulled out of URL parameters. 

This just changes `patch()` to use the name `data`, instead of `params`.