Proxy: escape explicit space in URI in default cache key. If the flag space_in_uri is set, the URI in HTTP upstream request is escaped to convert space to %20. However this flag is not checked while creating the default cache key. This leads to different cache keys for requests '/foo bar' and '/foo%20bar', while the upstream requests are identical. Additionally, the change fixes background cache updates when the client URI contains unescaped space. Default cache key in a subrequest is always based on escaped URI, while the main request may not escape it. As a result, background cache update subrequest may update a different cache entry.