fix unchecked conversion warnings in BulkService.java
fix unchecked conversion warnings in BulkService.java

naver/arcus-java-client,jam2in/arcus-java-client,jam2in/arcus-java-client,naver/arcus-java-client,whchoi83/arcus-java-client,whchoi83/arcus-java-client,jam2in/arcus-java-client,qeist/arcus-java-client,wskplho/arcus-java-client,whchoi83/arcus-java-client,wskplho/arcus-java-client,jam2in/arcus-java-client,naver/arcus-java-client,naver/arcus-java-client,wskplho/arcus-java-client,qeist/arcus-java-client,qeist/arcus-java-client