Reproduction steps:
1. Open project attached (Repro.zip).
2. Open scene "Test".
3. Press Play.
4. Wait till memory usage stabilizes.
5. Inspect how memory usage slowly increases. (does not start instantly, wait for 5-10 seconds)

Expected result: Memory usage will not increase.
Actual result Memory usage slowly increases.

Notes: Since 5.6.1p1 leak speed increased from 0-0.1 to 0.2-0.3 every second.

Reproduced on: 5.6.1p1, 2017.1.0f1, 2017.1.0b1.
Works fine on: 5.6.1f1.
Regression introduced in 5.6.1p1.

Forum thread: https://forum.unity3d.com/threads/am-i-going-crazy-big-memory-leak-in-networktransport-send.479295/

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Fixed in: 5.6.2p2, 2017.1.0f3 (not confirmed) and 2017.2.0b2