fix issue #29

[kevindragon]

修复了 issue #29

• [x] 测试了solr 7.2.0 (copy field)
• [x] 运行了com/hankcs/lucene/HighLighterTest.java测试

主要是覆盖了end方法，设置正确的end offset

    @Override
    public void end() throws IOException {
        offsetAtt.setOffset(totalOffset, totalOffset);
    }

[hankcs]

谢谢！