Bugfix: Run() fails for queries, add MarshalJSON()

The `Run()` method on the `QueryRequest` type would fail, since it would
encode the _inner_ body of the query to JSON and not the complete, outer
body (i.e. including the "body: {}" portion).

The commit also adds `MarshalJSON()` methods to both `QueryRequest` and
`AggregationRequest`, allowing them to implement the `json.Marshaller`
interface, and providing easier debugging of the library. A test
skeleton for this is also added.

aggregations.go

@@ -40,6 +40,10 @@ func (req *AggregationRequest) Map() map[string]interface{} {
 	}
 }
 
+func (req *AggregationRequest) MarshalJSON() ([]byte, error) {
+	return json.Marshal(req.Map())
+}
+
 func (req *AggregationRequest) Run(
 	api *elasticsearch.Client,
 	o ...func(*esapi.SearchRequest),