custom_filters_score with score_mode first doesn't return explanation correctly

[IoriH]

If score_mode is set to "first", explanation details inludes only details of boost filter.
When other score_mode is used, details contains details of both query and boost filter(s).

[martijnvg]

Nice catch! Indeed the explain of the query isn't included when score_mode=first is used. I'll add it.

[martijnvg]

Pushed.