Arduino to Thingspeak using WiFly-RN171 (Firmware 4.0)
I solved my problem by sending a GET request to a hosted PHP script which in turn updates Carriots by means of a Curl request.

My PHP Script

&lt;?php
$humidity=$_GET['humidity'];
$data = array("protocol"=&gt;"v2", //Compose JSON stream
              "at"=&gt; "now",
              "device"=&gt;"DEVICE_NAME",
              "data" =&gt; array ("humidity" =&gt; trim($humidity)),
              "checksum" =&gt; "");
$data_string = json_encode($data);

$ch = curl_init('https://api.carriots.com/status'); //Compose the HTTP request                                                                  
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'carriots.apikey: YOUR_API_KEY',
    'Content-Length: ' . strlen($data_string))
);       

$result = curl_exec($ch); //Send the stream


Note: 


I switched to Carriots from ThingSpeak due to personal preference.
This is by no means a standard way to send stream to Carriots.This
involves additional effort which slows down transmission of data but
I decided that this setup will be enough for a hobby project.
A good understanding of how POST requests are built should allow me
to skip the extra PHP script completely.