JENKINS-21594: Allow to upload file without wildcard

Commit 24a3b88[1] introduced a feature for wildcard uploads, but broke
the original behavior if your upload is configured with a straight
filename.

For instance:
Destination bucket: path/to/dir
Source: file.tar.gz

Would be uploaded as path/to/dir, missing completely the source file name.

This commit fixes this specific issue.

[1]
https://github.com/jenkinsci/s3-plugin/commit/24a3b88a7b9acb0532deabe686944e53747c49a9

Signed-off-by: Brice Figureau <brice@daysofwonder.com>
Merge pull request #21 from masterzen/fix/JENKINS-21594-no-wildcard-upload

JENKINS-21594: Allow to upload file without wildcard