Add riak_test for object list after deletes

Tests #654. The test currently fails, the next commit will fix it.
Don't skip past empty common prefixes

Fixes #654. If the list of so-far-matching common prefixes is empty, we
shouldn't try and skip past the last (sorted) common prefix. Otherwise,
we try and call `lists:last/1` on the empty list.
Never rewind when skipping keys in keylist

Fixes
[this](https://github.com/basho/riak_cs/pull/655#issuecomment-23309088)
issue comment. When skipping past a common prefix, we used to mistakenly
skip past only the last common prefix in the state record. There could,
however, be a later-sorted common prefix in the result set. So we now
skip past the exact common prefix that the last result is (if it is
one), instead of the last common prefix in the common prefixes field.
Include prefix in `common_prefix_from_key`
Add two more regression tests for #654

Based on feedback from @shino.
Merge pull request #655 from basho/bugfix/empty-object-list

Fix empty object list with delimiter