Cache: handling of cache files with long headers.

Cache: handling of cache files with long headers.

There are two possible situations which can lead to this: response was
cached with bigger proxy_buffer_size value (and nginx was restared since
then, i.e. shared memory zone content was lost), or due to the race in
the cache update code (see [1]) we've end up with fcn->body_start from
a different response stored in shared memory zone.

[1] http://mailman.nginx.org/pipermail/nginx-devel/2011-September/001287.html