code: public HttpRequest setSSLProtocol(String protocol) {
  		if (null == this.ssf) {
  			try {
  				this.ssf = SSLSocketFactoryBuilder.create().setProtocol(protocol).build();
  			} catch (Exception e) {
  				throw new HttpException(e);
  			}
  		}
  		return this;
  	}
docs: 设置HTTPS安全连接协议，只针对HTTPS请求，可以使用的协议包括：<br>
  
  <pre>
  1. TLSv1.2
  2. TLSv1.1
  3. SSLv3
  ...
  </pre>
  
  @see SSLSocketFactoryBuilder
  @param protocol 协议
  @return this