Added tests for UTF-8 validity of option values.

So far tests of the API and the set command checked only for ASCII values. Usage of UTF-8 should lead to no probems. The usage of UTF runes in the test value verifies this.
When adding stuf like the umlauts it has been ok for me, because I type them almost every day. But I felt somehow strange with the emojis too. So yes, I'll change it into unicode escapes.