In particular this fixes a bug where Content-Length was set incorecctly for disallowed
mime types.
Handler_Public::cached() set a Content-Length header before calling $cache->send().
If the mime type of the cached file is invalid DiskCache::send() returns an error
message instead, thus changing the size of the response.
For example nginx logs this as "[error] 7#7: *6864 upstream prematurely closed FastCGI
request while reading upstream".
