fix(mango): correct behavior of fields on _explain

[pgj]

The fields attribute on _explain might have the string value "all_fields" when the respective query parameter (fields) is not set by the user. This is to express that no projection of fields would happen on the returned documents.

The current behavior contradicts with the current contract, because by documentation, fields is an array of strings instead to provide information on the projected fields. The discrepancy here makes it hard to formalize this in OpenAPI which leads to problems in the development of SDKs, among others.

Change _explain to return [] for fields when it was not set through the query parameters. This is thought to be semantically equivalent to "all_fields" therefore this would not cause problems while preserving the promised type.

Thanks @ricellis for reporting this problem!

Testing recommendations

make eunit apps=mango

Users who based their solution on utilizing the "all_fields" response explicitly might need to change it to use [] instead. Otherwise they will not be affected by the change.

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Documentation changes were made in the src/docs folder

[nickva]

+1 (with a few grammar nits)

The main thing I checked was that previously fields: [] was not a thing and it meant all_fields anyway, as opposed some odd "I'd like no fields at all returned" query.