Inherit valid_unparsed_uri in cloned subrequests (ticket #1430).

Inherit valid_unparsed_uri in cloned subrequests (ticket #1430).

Inheriting this flag will make the cloned subrequest behave consistently with
the parent.  Specifically, the upstream HTTP request and cache key created by
the proxy module may depend directly on unparsed_uri if valid_unparsed_uri flag
is set.  Previously, the flag was zero for cloned requests, which could make
background update proxy a request different than its parent and cache the result
with a different key.  For example, if client URI contained the escaped slash
character %2F, it was used as is by the proxy module in the main request, but
was unescaped in the subrequests.

Similar problems exist in the slice module.