This is how I communicate with the webserver on the raspberry pi from Unity3d

CylonCoroutine.cs

using UnityEngine;
using System.Collections;
using System;

public class CylonCoroutine : MonoBehaviour {
	
	public string MoveType = "look_down";

	string server = "http://192.168.43.149:3000/";
	//string server = "http://192.168.1.25:3000/";
	// Use this for initialization
	void Start () {
		StartCoroutine (UpdateHead ());
	}
	
	// Update is called once per frame
	void Update () {
		
	}

	IEnumerator UpdateHead()
	{
		while (true) {
			if (!string.IsNullOrEmpty (MoveType)) {	
				//http://192.168.43.149:3000/api/robots/boxhead/commands/look_down
				var command = string.Empty;
				switch (MoveType) {
				case "look_up":
					command = "look_up";
					break;
				case "look_down":
					command = "look_down";
					break;
				case "look_left":
					command = "look_left";
					break;
				case "look_right":
					command = "look_right";
					break;
				default:
					command = string.Empty;
					break;
				}

				if (string.IsNullOrEmpty (command)) {
					yield return null;
				} else {
					var webcommand = server + "api/robots/boxhead/commands/" + MoveType;
					var www = new WWW (webcommand);
					MoveType = null;
					yield return www;

					if (string.IsNullOrEmpty (www.error)) {
						Debug.Log ("Return Value " + www.text);
					}
				}
			} else {
				yield return null;
			}
		}	
	}

	public IEnumerator UpdateBoxHead()
	{
			if (!string.IsNullOrEmpty(MoveType))
			{	
					//http://192.168.43.149:3000/api/robots/boxhead/commands/look_down
					var webcommand = "http://192.168.43.149:3000/api/robots/boxhead/commands/" + MoveType;
					var www = new WWW (webcommand);
					MoveType = null;
					yield return www;

					/*if (string.IsNullOrEmpty (www.error)) {
						Debug.Log ("Return Value " + www.text);
					}*/
			}

	}
}