Search in multivalue fields

[e-kolpakov]

Description: Documented search behavior of using contains predicate instead of equal_to if search value is singular and document value is multivalue one.
Background: the only supported search engine so far (elastic) analyses multivalues fields essentially resulting in "contains in" search using term filter/query, as opposed to "equals to" when non-multivalue field is used. This behavior considered a good syntax, so it was documented and covered with tests.
JIRA ticket: SOL-591

Also, I've took the responsibility of splitting single 1500+ line long test module into smaller modules. There are no changes in moved test suites except necessary modifications to patch calls.

[coveralls]

Coverage remained the same at 100.0% when pulling b5444b2 on ekolpakov/multivalue-search into 21ac6b0 on master.

[Kelketek]

@e-kolpakov Are these lyrics under a compatible license? I know it may seem silly, but it's something we probably need to be sure of.

[Kelketek]

@e-kolpakov Nevermind, I see this existed beforehand.

[Kelketek]

@e-kolpakov Aside from that question about the docs, this looks good to me. 👍

[e-kolpakov]

@martynjames you might want to review this (or assign someone else)

[martynjames]

Thanks @e-kolpakov - will look at it. Would've been easier if the separating of the tests into different files would have been separated from the actual changes you were implementing. Never mind about that now - but this is one case where having 2 different commits would actually make sense!

[martynjames]

👍 - Please next time consider separating the refactoring into a separate PR or at least a separate commit.

[e-kolpakov]

@martynjames sure, thank you for review.