GMS-2022-3357: Scrapy before v2.6.2 and v1.8.3 vulnerable to one proxy sending credentials to another

Because of request retries and redirects, the same request can be processed by downloader middlewares more than once, including both the built-in HTTP proxy downloader middleware and any third-party proxy-rotation downloader middleware. These third-party proxy-rotation downloader middlewares could change the proxy metadata of a request to a new value, but fail to remove the Proxy-Authentication header from the previous value of the proxy metadata, causing the credentials of one proxy to be leaked to a different proxy.