ES3: Unary operations on numeric strings not functioning

[cpeyer]

MYVAR='0Xf';MYVAR++; // = 0 expected: 15
MYVAR=''; MYVAR++; // = NaN expected: 0
MYVAR=new String('12345');MYVAR++; // = 0 expected: 12345
MYVAR=new String('12345');--MYVAR; // = -1 expected: 12344
// also doesn't return NaN when it should
MYVAR=new String('string');MYVAR--;MYVAR; // = -1 expected: NaN

ES3 Tests:
tests/spidermonkey/ecma/Expressions/11.3.1, 11.3.2, 11.4.x

[David-Sarah Hopwood]

Affects conformance of RefImpl? to ES3.1.