Couldn't send url encoded form params

The server receives the url encoded form params as single entry with param "key" populated with whole thing mentioned for "test.request.body" in raml file and param "value" as empty string. Like,

key = {"id": "...", "imageType": "...", "imageBase64Encoded": "..."}, value = ""

Note: I don't have any problem with RAML file.

Hookup file:
hooks.before('POST /v1/foo -> 200', function (test, done) {

```
test.request.headers = {
'Authorization': 'Bearer ' + '77aacdc2-49ea-32cd-8772-e90f3f3e1325',
'Content-Type': 'application/x-www-form-urlencoded'
};
test.request.body = {
id: '...',
imageType: 'image/png',
imageBase64Encoded: '....'
}
```

}