Proxy: escape explicit space in URI in default cache key.

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.