Changed Cache-Control header to be sent via err_headers_out

This pull request is in response to https://github.com/bmuller/mod_auth_openid/issues/44.  Cache-Control headers were not being sent out.  Using r->err_headers_out instead of r->headers_out properly sets the Cache-Control.  I am using this fork in place and it has corrected the issue in our production system.  Cache-Control headers are now observed.  

SB