Add tests for bugs reported against riak_kv: 917, 918

@javajolt found two glaring bugs with security and filed tickets (basho/riak_kv#917, basho/riak_kv#918); these new tests should detect any regressions.

@bowrocker, please give this branch a try against your bug fix for basho/riak_kv#918. It should still fail on the props test (waiting for a fix on basho/riak_kv#917), but should pass the non-existent key test.