Currently we only have tests that tests when one of these combination options (weights or method) are specified. We need to add tests (unit and js) where both are specified when the combination.method is avg. (Note that combination.weights is disallowed when combination.expression is specified).

For jstests, we should just be able to add more tests with weights specified to score_fusion_combination_test.js

Note: right now with combination.method.expression we need to put the 'docs.' prefix in front of collection/document variables, which we later plan to remove. We can still write these tests before that changes goes through, and later update this test after they are done.