Upstream: even better handling of invalid headers in cache files.

Upstream: even better handling of invalid headers in cache files.

When parsing of headers in a cache file fails, already parsed headers
need to be cleared, and protocol state needs to be reinitialized.  To do
so, u->request_sent is now set to ensure ngx_http_upstream_reinit() will
be called.

This change complements improvements in 46ddff109e72.