Fixed String.prototype.trimEnd() with unicode string.

Fixed String.prototype.trimEnd() with unicode string.

Previously, when the method was invoked with a string consisting of space
characters and at least one of them was a Unicode space separator (code
point above 127) it returned invalid string value with non-zero size
but zero length.

The fix is to update the size of the resulting string appropriately.

This closes #569 issue on Github.