public Header setContentType(String contentType) {
        if (contentType == null) {
            contentType = MediaType.APPLICATION_JSON;
        }
        return addParam(HttpHeaderConsts.CONTENT_TYPE, contentType);
    }
@Test
    void testSetContentType() {
        Header header = Header.newInstance();
        header.setContentType(null);
        assertEquals(MediaType.APPLICATION_JSON, header.getValue(HttpHeaderConsts.CONTENT_TYPE));
        header.setContentType(MediaType.MULTIPART_FORM_DATA);
        assertEquals(MediaType.MULTIPART_FORM_DATA, header.getValue(HttpHeaderConsts.CONTENT_TYPE));
    }