Not able to specify missing_value in aggregation

`aggregations: {
"aggregation_name": {
"terms": {
"field": "field_name",
"size": number,
"missing": "field value"
}
}
}`

ES allows to assign a value to records with missing values, currently not possible to generate this query.

The issue does not name files or tests. Start by locating the Ruby aggregation implementation for terms and its existing serialization tests, then compare their output with the Elasticsearch example in the issue; the work is done when a terms aggregation can emit the requested missing value.

Written by the indexing model from the issue text.