JAVA-2047: Use default DBObject Codec for the aggregate helper methods. This is in line with the behavior of the 2.x branch.

Author: jyemin

driver/src/main/com/mongodb/DBCollection.java

@@ -1368,7 +1368,7 @@ private Cursor aggregate(final List<? extends DBObject> pipeline, final Aggregat
                 return null;
             }
         } else {
-            AggregateOperation<DBObject> operation = new AggregateOperation<DBObject>(getNamespace(), stages, objectCodec)
+            AggregateOperation<DBObject> operation = new AggregateOperation<DBObject>(getNamespace(), stages, getDefaultDBObjectCodec())
                                                          .readConcern(getReadConcern())
                                                          .maxTime(options.getMaxTime(MILLISECONDS), MILLISECONDS)
                                                          .allowDiskUse(options.getAllowDiskUse())

driver/src/test/functional/com/mongodb/DBCollectionSpecification.groovy

@@ -325,7 +325,7 @@ class DBCollectionSpecification extends Specification {
         aggregationOutput.results() == [dbObject]
         expect executor.getReadOperation(), isTheSameAs(new AggregateOperation(collection.getNamespace(),
                                                                                [new BsonDocument('$match', new BsonDocument())],
-                                                                               collection.getObjectCodec())
+                                                                               collection.getDefaultDBObjectCodec())
                                                                 .readConcern(ReadConcern.MAJORITY)
                                                                 .useCursor(false)
         )