How to reproduce:
1. Create new project file and switch Platform to WebGL
2. Create and assign to GameObject new c# code:

public class NewBehaviourScript : MonoBehaviour {

void Start () {
using(var uwr = new UnityWebRequest("/play/something"))
{
Debug.Log("Formatted url: " + uwr.url);
}
}
}

3. Press Build & Run

Expected result: Formatted url: http://localhost:<port number>/play/something
Actual result: Formatted url: http://localhost/play/something

Reproduced with: 5.6.4p2, 2017.1.2p3, 2017.2.0p2, 2017.3.0b9, 2018.1.0a4

Workaround:

using(var uwr = new UnityWebRequest(Application.absoluteURL + "relative/url"))
{
Debug.Log("Formatted url: " + uwr.url);
}