SignificantTerms Aggregation is missing the background_filter parameter

[kohner86]

The background_filter parameter for SignificantTerms aggregates seems to be missing although it is documented here:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-significantterms-aggregation.html

-Lee

[gmarz]

@kohner86 Thanks for reporting 👍