public Site addHeader(String key, String value) {
        headers.put(key, value);
        return this;
    }
Put an Http header for downloader. <br>
Use {@link #addCookie(String, String)} for cookie and {@link #setUserAgent(String)} for user-agent. <br>

@param key   key of http header, there are some keys constant in {@link HttpConstant.Header}
@param value value of header
@return this
webmagic-core/src/main/java/us/codecraft/webmagic/Site.java