Check whether SL1 strings are Unicode

[jackmcbarn]

Distinguish between an omitted value and a present default value by checking what type of string it is (omitted values are normal strings, and present default values are Unicode strings).

[CensoredUsername]

I suppose the same would then apply to the ast.tag fix from #29? Could you add that before I merge it in?

edit: although I suppose it doesn't apply directly since the default value for tag is None, I'd like for the handling of tag in screendecompiler and sl2decompiler to be the same.

[jackmcbarn]

I don't think it would apply there, since that uses the Python constant None, not the string "None".

[CensoredUsername]

The same goes for ast.tag in screen language 1 screens, it is by default the value None, not the string "None", so then I'd say change the suggested SL1 check back into "if ast.tag:"

[jackmcbarn]

Oh, right. Fixed.