[BUG] WeChat does not export HTTPClient and cannot be customized

Hi

I tried to use WeChat, but found that I could not modify the default HTTPClient

See: https://github.com/markbates/goth/blob/5d0f51eacaf316657eeea303a75d36ca3a32c269/providers/wechat/wechat.go#L27