S4cmd -n, --dry-run does not seem to always work with delete operation

I've been using `s4cmd del -n` recently for dry-runs before deleting objects, and have seen it actually deleting the objects. The behavior seems sporadic. I've seen it with and without `-r, --recursive` and with and without wildcards in the path, but it appears that perhaps this behavior occurs more frequently with recursion and wildcards?

I'm using version 2.0.1

```
$ pip show s4cmd
Name: s4cmd
Version: 2.0.1
Summary: Super S3 command line tool
Home-page: https://github.com/bloomreach/s4cmd
Author: Chou-han Yang
Author-email: UNKNOWN
License: http://www.apache.org/licenses/LICENSE-2.0
Location: /Users/seanq/anaconda/lib/python2.7/site-packages
Requires: boto3, pytz
```

Start by reproducing `s4cmd del -n` with and without `--recursive` and wildcards on the reported version, then trace the delete command's dry-run handling. Done means each of these forms consistently reports intended deletions without deleting any objects; the issue names no source file or test.

Written by the indexing model from the issue text.