code: protected String getProtocolByHttpContentType(HttpServletRequest httpServletRequest) {
          if (StringUtil.isEmpty(httpServletRequest.getContentType())) {
              throw new InvalidRequestException("Rpc protocol invalid");
          }
          String protocol = httpServletRequest.getContentType().split(";")[0];
          if (protocol == null) {
              protocol = NaviCommonConstant.DEFAULT_PROTOCAL_CONTENT_TYPE;
              if (LOG.isDebugEnabled()) {
                  LOG.debug("Content-type set to default value "
                          + NaviCommonConstant.DEFAULT_PROTOCAL_CONTENT_TYPE);
              }
          } else {
              protocol = protocol.toLowerCase();
          }
          return protocol;
      }
docs: 获取http请求的request中的content-type，如没有设置则设置为默认的
  {@link com.baidu.beidou.navi.constant.NaviCommonConstant#DEFAULT_PROTOCAL_CONTENT_TYPE}
  
  @param httpServletRequest
  @return