send requests using POST method and body formatted to JSON

Solves errors like:

```
OmnitureClient::Exceptions::RequestInvalid: {"error":"element_id_missing","error_description":"Element id must be specified","error_uri":"https:\/\/marketing.adobe.com\/developer"}
```

They were probably caused by GET method getting cut by webserver. According to RFC there is no limit, but "id missing" errors indicates that.