Adding headers to included files / sub-scopes drops those set earlier

[colans]

From the documentation:

There could be several add_header directives. These directives are inherited from the previous level if and only if there are no add_header directives defined on the current level.

So we should keep these in nginx.conf. Currently, the ones causing me problems are the following:

• add_header Strict-Transport-Security "max-age=7200";

...and several in apps/drupal/microcache_fcgi.conf