feat(matchedData): add discardUndefined option when selecting fields

[Vladyslav531]

Description

Add ignoreUndefined to includeOptionals in matchedData

ignoreUndefined allows user to pass optional values which aren't undefined like null and falsy values.
It keeps current behaviour and allow user to customize matchedData.

Closes #1048
Closes #1175
Closes #1273

To-do list

• I have added tests for what I changed.

• This pull request is ready to merge.

[Vladyslav531]

@gustavohenke Could you check this PR please?

[coveralls]

coverage: 100.0%. remained the same
when pulling 5e783d3 on Vladyslav531:feat/ignoreUndefined
into ffd7994 on express-validator:master.

[fedeci]

Thanks! Looks like a good new feature, there are some changes to publish this in a minor version.

[fedeci]

Thanks! Looks like a good new feature, there are some changes to publish this in a minor version.

[fedeci]

Also, docs should be updated too once we are good to go with the implementation and the new API.

[Vladyslav531]

Also, docs should be updated too once we are good to go with the implementation and the new API.

I update docs, you can check

[Vladyslav531]

@fedeci I updated code to keep signature of context. You can check again

[fedeci]

I also changed my mind on the naming of ignoreUndefined. It may be confusing. What about discardUndefined?
/cc @gustavohenke

[Vladyslav531]

I also changed my mind on the naming of ignoreUndefined. It may be confusing. What about discardUndefined? /cc @gustavohenke

I think it is also good name.