Gem: POST method needs "body" rather then "parameters" parameter

After being unsuccessful in doing a POST request as per the documentation I dug into the code in the current version of the gem on rubygems (unirest-1.0.8) and discovered that while the documentation states that the post method takes a "parameters" parameter with the POST body, the actual method is expecting a "body" parameter.

```
def self.post(url, headers = {}, body = nil, &callback)
return HttpClient.request(:post, url, headers, body, &callback)
end
```

Start with the POST method signature shown in the issue and compare it with the gem documentation for POST requests. Update the documented parameter from "parameters" to "body", then verify that the example matches the method signature.

Written by the indexing model from the issue text.