Merge of r4275, r4276, r4278, r4279:

Merge of r4275, r4276, r4278, r4279:

Fixes for proxy_set_header, fastcgi/scgi/uwsgi_param inheritance:

*) Fixed proxy_set_header inheritance with proxy_cache (ticket #45).

   Headers cleared with cache enabled (If-Modified-Since etc.) might be
   cleared in unrelated servers/locations without proxy_cache enabled
   if proxy_cache was used in some server/location.

   Example config which triggered the problem:

       proxy_set_header X-Test "test";
       server { location /1 { proxy_cache name; proxy_pass ... } }
       server { location /2 { proxy_pass ... } }

   Another one:

       server {
           proxy_cache name;
           location /1 { proxy_pass ... }
           location /2 { proxy_cache off; proxy_pass ... }
       }

   In both cases If-Modified-Since header wasn't sent to backend in
   location /2.

   Fix is to not modify conf->headers_source, but instead merge user-supplied
   headers from conf->headers_source and default headers (either cache or not)
   into separate headers_merged array.

*) Fixed proxy_set_header inheritance with proxy_set_body.

*) Separate functions to merge fastcgi/scgi/uwsgi params.
   No functional changes.

*) 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.