Upstream: skip empty cache headers.

Notably this fixes HTTP_IF_MODIFIED_SINCE which was always sent with
cache enabled in fastcgi/scgi/uwsgi after 43ccaf8e8728.

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 51672e2..4cbe51c 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2769,7 +2769,7 @@
 
             s->key = h->key;
             s->value = h->value;
-            s->skip_empty = 0;
+            s->skip_empty = 1;
 
         next:
 

src/http/modules/ngx_http_scgi_module.c

diff --git a/src/http/modules/ngx_http_scgi_module.c b/src/http/modules/ngx_http_scgi_module.c
index 9163c6b..d396b27 100644
--- a/src/http/modules/ngx_http_scgi_module.c
+++ b/src/http/modules/ngx_http_scgi_module.c
@@ -1506,7 +1506,7 @@
 
             s->key = h->key;
             s->value = h->value;
-            s->skip_empty = 0;
+            s->skip_empty = 1;
 
         next:
 

src/http/modules/ngx_http_uwsgi_module.c

diff --git a/src/http/modules/ngx_http_uwsgi_module.c b/src/http/modules/ngx_http_uwsgi_module.c
index 9c5d762..c19c7c7 100644
--- a/src/http/modules/ngx_http_uwsgi_module.c
+++ b/src/http/modules/ngx_http_uwsgi_module.c
@@ -1548,7 +1548,7 @@
 
             s->key = h->key;
             s->value = h->value;
-            s->skip_empty = 0;
+            s->skip_empty = 1;
 
         next: