code: @Test public void testUpdateHeader(){
    HeaderGroup headergroup=new HeaderGroup();
    Header header1=new BasicHeader("name1","value1");
    Header header2=new BasicHeader("name2","value2");
    Header header3=new BasicHeader("name3","value3");
    headergroup.addHeader(header1);
    headergroup.addHeader(header2);
    headergroup.addHeader(header3);
    headergroup.updateHeader(new BasicHeader("name2","newvalue"));
    headergroup.updateHeader(new BasicHeader("name4","value4"));
    headergroup.updateHeader(null);
    Assert.assertEquals(4,headergroup.getAllHeaders().length);
    Assert.assertEquals("newvalue",headergroup.getFirstHeader("name2").getValue());
  }
  