fix: Make NetworkManager.Shutdown idempotent

[simon-lemay-unity]

If calling Shutdown on a NetworkManager that's already shut down, the m_ShuttingDown flag would still be set to true. But if the NetworkManager is not running then ShutdownInternal won't get an opportunity to run and clear that flag until the next time the NetworkManager is started.

The result is that calling Shutdown on a NetworkManager that's not running will cause an immediate shutdown the next time it does run. That's unlikely to be the desired behavior here.

The fix is to not set m_ShuttingDown if the NetworkManager is not running. This makes Shutdown idempotent; calling it again multiple times produces no effect.

Changelog

• Fixed: Fixed an issue where calling Shutdown on a NetworkManager that was already shut down would cause an immediate shutdown the next time it was started (basically the fix makes Shutdown idempotent).

Testing and Documentation

• Includes integration test.
• No documentation changes or additions were necessary.

[simon-lemay-unity]

Note that this entry had been accidently put under the 1.0.0-pre.7 section.

[jeffreyrainy]

Just linking: This should fix #1595