ipro-lff: Prevent handing out 200/OK when we can't rewrite in place

- Reorder setting the status code so we actually change the
  ResponseHeaders that we are using to serve
- Prevent non-error http status codes from being served up.

PSOL patch:
```diff
--- ../rewriter/in_place_rewrite_context.cc	(revision 4020)
+++ ../rewriter/in_place_rewrite_context.cc	(working copy)
@@ -127,9 +127,20 @@
       // to us.
       streaming_ = false;
       set_request_headers(NULL);
+      // If we cannot rewrite in-place, we should not serve a 200/OK.
+      // Serve kNotFound instead to fall back to the server's native
+      // method of serving the url and indicate we don't want it recorded
+      // either.
+      // TODO(oschaaf): When the resource is loaded via LoadFromFile,
+      // this code gets repeatedly hit for the same url. That might
+      // become a drag when the file in question is very large and popular.
+      // Marking urls that end up in this flow as not cachable in the http cache
+      // at this point had no effect (for the LoadFromFile flow at least).
+      if (!response_headers()->IsErrorStatus()) {
+	response_headers()->set_status_code(HttpStatus::kNotFound);
+      }
       set_response_headers(NULL);
       set_extra_response_headers(NULL);
-      response_headers()->set_status_code(HttpStatus::kNotFound);
       SharedAsyncFetch::HandleDone(false);
     }
   }
```