hotfix(core) ensure X-Kong-Upstream-Status header is injected
hotfix(core) ensure X-Kong-Upstream-Status header is injected

The hotfix commit 75e071b (#3419) changes the configuration property from
`headers` to `enabled_headers`.

The commit did not update a conditional statement introduced in
60c335e6 (#3263). This commit corrects the check from `headers` to
`enabled_headers`.

This was caught by the test suite and hence no regression test
has been added.

From #3433
Kong/kong,Mashape/kong,Kong/kong,Kong/kong