Fixed fastcgi/scgi/uwsgi_param inheritance.

The following problems were fixed:

1. Directive fastcgi_cache affected headers sent to backends in unrelated
   servers / locations (see ticket #45).

2. If-Unmodified-Since, If-Match and If-Range headers were sent to backends
   if fastcgi_cache was used.

3. Cache-related headers were sent to backends if there were no fastcgi_param
   directives and fastcgi_cache was used at server level.