Steps to reproduce:

1) Download attached project 'HttpWebRequest.zip' and open in Unity
2) Create a new scene
3) Drag and drop 'NewBehaviourScript' on MainCamera
4) Build and run this scene on a device
5) Open Command Prompt and run 'adb logcat -s Unity'

Note: You should receive this error in logs:
02-17 13:06:14.724 15095 15180 D Unity : Platform assembly: /data/app/com.Company.HttpWebRequest-1/base.apk/assets/bin/Data/Managed/Mono.Security.dll (this message is harmless)
02-17 13:06:15.512 15095 15186 D Unity : Unable to lookup library path for '/System/Library/Frameworks/Security.framework/Security', native render plugin support disabled.
02-17 13:06:15.512 15095 15186 E Unity : Unable to find /System/Library/Frameworks/Security.framework/Security
02-17 13:06:16.112 15095 15190 D Unity : Setting up 1 worker threads for Enlighten.
02-17 13:06:16.113 15095 15191 D Unity : Thread -> id: ffffffffc5281920 -> priority: 1
02-17 13:06:49.077 15095 15111 D Unity : PlayerConnection accepted from [10.37.35.38] handle:0x6b

Reproduced with:
5.2.5f1, 5.3.7p4, 5.4.4p3, 5.5.1p4, 5.6.0b9

DUT:
Samsung SM G930F, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/heroltexx/herolte:7.0/NRD90M/G930FXXU1ZPK3:user/release-keys

Samsung SM G900F (Galaxy S5), OS:6.0.1, CPU:armeabi-v7a, GPU:Adreno (TM) 330, Build:samsung/kltexx/klte:6.0.1/MMB29M/G900FXXS1CQAV:user/release-keys

Xiaomi MI 5, OS:6.0.1, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:Xiaomi/gemini/gemini:6.0.1/MXB48T/V8.1.2.0.MAAMIDI:user/release-keys

Samsung GT I8190, OS:4.1.2, CPU:armeabi-v7a, GPU:Mali-400 MP, Build:samsung/goldenxx/golden:4.1.2/JZO54K/I8190XXAMG4:user/release-keys