Steps to reproduce:
1. Download Charles software or something that can simulate poor internet connection (https://www.charlesproxy.com/)
2. Open "WebRequestTest.zip" attached project
3. Enable Throttling with user described settings on Charles software
4. Enter play mode
5. Click "J" , "K" , "L" buttons to observe each type of request
6. Only one that completes is the "WebClient Request" after pressing "J"

Expected: Even with the poor connection, the request should not get stuck, take a bit more time maybe, but complete eventually
Actual: WWW and UnityWebRequest get either entirely stuck or partially download the data and stop

Reproduced in 5.5.3p3, 5.6.0p1, 5.6.1p1, 2017.1.0b6