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.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 985d633..1f6a8fe 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -584,6 +584,7 @@
                 r->cached = 0;
                 u->buffer.start = NULL;
                 u->cache_status = NGX_HTTP_CACHE_MISS;
+                u->request_sent = 1;
             }
 
             if (ngx_http_upstream_cache_background_update(r, u) != NGX_OK) {