Fixed riak_object RpbPair bugs

There were two issues here.

First, only the value portion of the pair was being copied.

Second, the RpbPair structs weren't being initialized, which would cause the
client to core during PB serialization. This would affect setting indexes, links,
or user metadata on a riak_object.
Merge branch 'feature/bch/add-2i' of github.com:basho/riak-c-client into feature/bch/add-2i