Add deferred call to close DocumentResult's cursor

GODRIVER-373

Change-Id: I7addfb359952a5abfccd40c59bf294d3e30be16a

Author: mattChiaravalloti

.errcheck-excludes

@@ -4,6 +4,7 @@
 (*github.com/mongodb/mongo-go-driver/core/topology.Server).Close
 (*github.com/mongodb/mongo-go-driver/core/connection.pool).closeConnection
 (github.com/mongodb/mongo-go-driver/core/wiremessage.ReadWriteCloser).Close
+(github.com/mongodb/mongo-go-driver/mongo.Cursor).Close
 (net.Conn).Close
 encoding/pem.Encode
 fmt.Fprintf

mongo/document_result.go

@@ -40,6 +40,7 @@ func (dr *DocumentResult) Decode(v interface{}) error {
 		}
 		return bson.Unmarshal(dr.rdr, v)
 	case dr.cur != nil:
+		defer dr.cur.Close(context.TODO())
 		if !dr.cur.Next(context.TODO()) {
 			if err := dr.cur.Err(); err != nil {
 				return err