Upstream: unconditional parsing of last_modified_time.

This fixes at least the following cases, where no last_modified_time
(assuming caching is not enabled) resulted in incorrect behaviour:

- slice filter and If-Range requests (ticket #1357);
- If-Range requests with proxy_force_ranges;
- expires modified.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 6a2b322..73a5882 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4390,15 +4390,8 @@
     u = r->upstream;
 
     u->headers_in.last_modified = h;
-
-#if (NGX_HTTP_CACHE)
-
-    if (u->cacheable) {
-        u->headers_in.last_modified_time = ngx_parse_http_time(h->value.data,
-                                                               h->value.len);
-    }
-
-#endif
+    u->headers_in.last_modified_time = ngx_parse_http_time(h->value.data,
+                                                           h->value.len);
 
     return NGX_OK;
 }
@@ -4940,15 +4933,8 @@
     *ho = *h;
 
     r->headers_out.last_modified = ho;
-
-#if (NGX_HTTP_CACHE)
-
-    if (r->upstream->cacheable) {
-        r->headers_out.last_modified_time =
+    r->headers_out.last_modified_time =
                                     r->upstream->headers_in.last_modified_time;
-    }
-
-#endif
 
     return NGX_OK;
 }