feat: conform to handler function signature

[eduardoboucas]

Which problem is this pull request solving?

This PR changes the signature of the wrapped function to only be a Promise if the wrapped method is also a Promise. The goal is to provide a better support for the legacy callback syntax.

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[eduardoboucas]

@erezrokah Is this what you had in mind in b3a1933#r47416097?

I wonder if this check should be (execution instanceof Promise || execution.then !== undefined)? Or even (execution !== undefined), because if you're returning something from the handler it probably means you're not using the callback syntax.

When it's unclear, I think we should lean towards treating it as an asynchronous handler since the callback syntax is now considered legacy.

[erezrokah]

Yes exactly - we could use https://github.com/then/is-promise to make it work with non native promise libs like http://bluebirdjs.com/.

I can see someone returning a value from a callback based pattern by mistake...

[eduardoboucas]

Done in ed0e801.

[erezrokah]

feel free to disregard my comment

[erezrokah]

I'm not sure if we should return execution if this is not a promise just for the sake of keeping the original function behavior.

Very much and edge case of someone returning a value from a callback based pattern

[eduardoboucas]

Done in 570d001.

I'm all for keeping this wrapper as little intrusive as possible, so this makes perfect sense to me. Thanks!