如何在Scrapy请求中发送自定义标头？

import scrapy

class MySpider(scrapy.Spider):
    name = 'myspider'

    def start_requests(self):
        headers = {
            'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3',
            'Referer': 'http://www.example.com'
        }
        yield scrapy.Request(url='http://www.example.com', headers=headers, callback=self.parse)

    def parse(self, response):
        # 解析响应数据
        pass