Made customSanitizer await for async functions

[NathanPB]

I made this little tweak to make possible use customSanitize in async environments. I did not made a proper error handler because I don't know how to integrate that with the validators, so I don't think the work of this feature is done, that's just a draft. This is related to #784

An example use case for this would be: body('user').customSanitizer(async(id) => await users.findById(id))

Also made the tests for that

[coveralls]

Coverage remained the same at 100.0% when pulling 3686317 on NathanPB:feature/await-sanitizer into ef31f67 on express-validator:master.

[gustavohenke]

Draft is already looking good to me!
What do you suggest that we do with the errors?

[NathanPB]

What do you suggest that we do with the errors?

I'd suggest automatically send a 500 http error code and abort the request at all.

Adding the error to the error list given by validationResult(req) seems wrong because usually people just send a 400 when there are errors present, and a 400 error code would not be the case for errors thrown in the sanitizers (5xx instead)

[gustavohenke]

Cool! I think that an error will already cascade up to the route definition, so this LGTM as is.

[gustavohenke]

Out in v6.7.0!

[wwei-flux]

Thanks for the feature. Should the document at https://express-validator.github.io/docs/custom-validators-sanitizers.html#custom-sanitizers be updated?

[fedeci]

Sure @wwei-flux, you can raise a PR if you want!