make 'mruby_cache' enabled in (post_read|server_rewrite)_handler

Even if mruby_cache is set on in location context,
it is not enabled in (post_read|server_rewrite)_handler.
In these handlers, 'mruby_cache' must be set in server context.
Merge pull request #24 from cubicdaiya/issues/mruby_cached_is_not_enabled

'mruby_cache' is not enabled in (post_read|server_rewrite)_handler