nginx: Removes problematic performance improvement

In an attempt to boost the performance the headers were being checked in two
different situations, the first if SecResponseBodyAccess was Off and in a
second situation if SecResponseBodyAccess was set to On. This makes sense
however functionalities such as content injection demands the body even if
SecResponseBodyAccess was not enabled. This patch removes the first scenario,
leaving just the second one. After this modification the following regression
tests started to pass:

from: regression/action/10-append-prepend.t
  1) action - append content: passed
  2) action - prepend content: passed