Avoid Select when no boxing required

[YohDeadfall]

What is the current behavior?

Always instantiate an instance of Select observer.

What is the updated/expected behavior with this PR?

Don't create any Select if the binding value type is a reference type.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Shouldn't but if the observable implements IObservable<object> then this interface will be used instead of covariance.

Alternative solutions

If we don't care about observables implementing IObservable<object> differently than IObservable<T> then the code can be rewritten to simply use as cast and use the result. It can be helpful for smart observables handling boxing internally, but that I assume is pretty rare.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051584-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051862-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

A simple but nice optimization.
LGTM, thank you!

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052381-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]