Fix String::__boot when smart strings are disabled

[tobil4sk]

Can be verified by running this code with -D disable-unicode-strings.

function main() {
	trace(StringTools.fastCodeAt(String.fromCharCode(128), 0));
	trace(StringTools.fastCodeAt(String.fromCharCode(255), 0));
}

The current (incorrect) output is:

src/Main.hx:2: 194
src/Main.hx:3: 195

With the patch, the output is correct:

src/Main.hx:2: 128
src/Main.hx:3: 255