Added missing conditional

The logic here was meant to have the PB client behavior match the HTTP
client behavior when only fetching the meta and siblings were found to
be present. The conditional for seeing if the 'head' parameter had been
set was missing, causing every fetch that had siblings to be repeated.
This actually isn't necessarity with PB, but I don't want to change this
behavior now in case someone is relying on it.
Removed second fetch entirely

Per our discussion, I've removed this entirely. There is simply no
reason to do a second fetch with the PB client. Metadata for all
siblings is returned in the first fetch if `head` was specified (tested
to make sure).
Removed test for removed behavior

Since we no longer do a second fetch for values in the PBClientAdapter
... we shouldn't try and test that.
Merge pull request #193 from basho/gh192-pb-fetches-twice

Removed second fetch