WARN Connection header illegal in HTTP/2: connection about spider

[j-mendez]

@saito-kosuke A config to determine if the connection should be kept alive is needed to prevent the warning. It should be safe to ignore since the client ignores the headers internally. Later on I was thinking it would make sense to add a duel client that can handle http1 and http2 depending on the website and the host network. A config for website.configuration.http2_prior_knowledge was added if you know the website has http2 support to prevent using http1 connections. 9063c00