코드 질문

package basic.practice.servletV1.response;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Collection;
import java.util.Enumeration;

/**
 * response header 조회하기
 */
@WebServlet(name = "responseHeaderServlet", urlPatterns = "/response-header")
public class ResponseHeaderServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("ResponseHeaderServlet.service");
        response.setContentType("text/plain");
        System.out.println(response.getHeader("Content-Type"));
    }
}

두번째 출력값이 널이 나오는 이유가 뭔가요??