Use match_all as default filter in filtered query

In the Elasticsearch JSON search DSL, using a filtered query without a
query will use the match_all query by default. This change mimics that
behaviour in the Scala DSL, which avoids an exception when not
specifying a query.

Fixes #186
Merge pull request #191 from fredrikroos/matchall-default

Use match_all as default filter in filtered query