ReadStream: never siphon if CacheMaxSize <= 0

If the user has not set a CacheMaxSize >0, we should never attempt to
start siphoning values while reading objects.
ReadStream: use implicit cache check instead of checking CacheSizeMax
Drop superfluous pointer on closingReader method receiver
Merge pull request #21 from jonboulle/nocache

ReadStream: never siphon if CacheMaxSize <= 0