Added a testcase that an IllegalArgumentException for invalid keys

[PaulWoitaschek]

Currently simple-store provides no checking on the naming of it's keys. Instead, when sth. like ./ is used as a key, the store does unexpected things (moving wrong files etc.). This PR adds a testcase that simply expects an IllegalArgumentException to be thrown because that's what I would expect from the library to do in these cases.

[CLAassistant]

All committers have signed the CLA.

[kurtisnelson]

I approve, can you add the check to the library?