$network.request with method POST (appears to) disregard the "content_type": "json"

the action below on iOS sends (correctly) JSON encoded data of the form

```
print request.data
{"object_type":"news","action_type":"like","object_id":"42"}
```
However, on Android the data is form encoded

```
print request.data
object_id=42&object_type=news&action_type=like
```

```
"action": {
"type": "$network.request",
"options": {
"url": "https://kadist-ios.herokuapp.com/action",
"method": "post",
"content_type": "json",
"data": {
"action_type": "like",
"object_type": "news",
"object_id": "{{$get.permalink}}"
}
}
}
```

Start by tracing the Android implementation of the $network.request action for POST requests and its handling of the content_type option. Compare the request serialization with the iOS behavior shown in the issue; done means a POST using content_type "json" sends a JSON body on Android as well.

Written by the indexing model from the issue text.