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.
I *personally* prefer ascii only source text (I realize go does specify source is UTF-8), and using unicode escapes to do non-standard characters.
I believe the last time this came up we went with "lets use ascii for our source" when someone wanted a variable named Façade. I'd probably be stricter about variables/functions rather than contents of strings, though.
So while I'd still recommend unicode escapes, I wouldn't require them.