make deep NaN === deep NaN for computed values when using compareStructural

[Ulmanb]

There seemed to be an inconsistent behavior where NaN === NaN but deep NaN !== NaN.
I think @benjamingr talked with you about it.

PR checklist:

• Added unit tests
• Verified that there is no significant performance drop (npm run perf) - it does seem to add around 1-3 second on my machine, not sure if this is significant.

[coveralls]

Coverage increased (+0.002%) to 94.653% when pulling 6052a7c on Ulmanb:master into 7fdf5f3 on mobxjs:master.

[benjamingr]

@mweststrate this is the one we discussed.

[mweststrate]

Thanks for noticing & PR, @Ulmanb & @benjamingr!