Fix lifetime issue with Deferred.

[danielt1263]

This is in response to PR #2533. It includes the test that was added in that PR along with the fix.

[freak4pc]

Thanks both @danielt1263 & @nikolaykasyanov
The fix looks good but I'm quite worried about changing the behavior (even if it's wrong) with a patch / minor release. Not sure it's a huge issue but let me think about it a bit for a day or two :)

If you have large code-bases it might be worth running it to confirm it works as expected at those scales.

[danielt1263]

I have some largish code bases, but I almost never use deferred in my RxSwift code. (I use it all the time in Combine, but not Rx). So they wouldn't be good tests.

[nikolaykasyanov]

I did it a bit of smoke testing and manual review (fairly big code base, around 20 instances of .deferred only though, if we don't count RxSwift itself), the change looks good from our side.