DATAMONGO-2410 - Polishing.

Simplify cast. Extend test with DBObject interface.

Original pull request: #813.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -253,7 +253,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				return (S) new BasicDBObject((Document) bson);
 			}
 
-			return (S) DBObject.class.cast(bson);
+			return (S) bson;
 		}
 
 		if (typeToUse.isCollectionLike() && bson instanceof List) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -20,6 +20,7 @@ import static org.assertj.core.api.Assertions.*;
 import static org.mockito.Mockito.*;
 import static org.springframework.data.mongodb.core.DocumentTestUtils.*;
 
+import com.mongodb.DBObject;
 import lombok.EqualsAndHashCode;
 import lombok.RequiredArgsConstructor;
 
@@ -2070,9 +2071,11 @@ public class MappingMongoConverterUnitTests {
 
 	@Test // DATAMONGO-2410
 	public void shouldAllowReadingBackDbObject() {
-		
+
 		assertThat(converter.read(BasicDBObject.class, new org.bson.Document("property", "value")))
 				.isEqualTo(new BasicDBObject("property", "value"));
+		assertThat(converter.read(DBObject.class, new org.bson.Document("property", "value")))
+				.isEqualTo(new BasicDBObject("property", "value"));
 	}
 
 	static class GenericType<T> {