In HPACK, specifying the index of an referenced entry dereferences it.

Found a slight spec gap and corrected it.

http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-05#section-3.2.1
```
   An _indexed representation_ with an index value of 0 entails the
   following actions:

   o  The reference set is emptied.

   An _indexed representation_ corresponding to an entry _present_ in
   the reference set entails the following actions:

   o  The entry is removed from the reference set.
```