More Like This Query: Values of a multi-value fields are compared at the same level

Previously, More Like This would create a new mlt query for each value of a
multi-value field. This could result in all the values of the field to be
selected, which defeats the purpose of More Like This. Instead, the correct
behavior is to generate only one mlt query for all the values of the field.
This commit provides the correct behavior for More Like This DSL. The fix for
More Like This API will be coming in another commit.