Fix bug indexOf. when arr is number, it was return 0

[hokaccha]

Not supported Array#indexOf browser(under IE8), this test is failed.

err(function () {
  expect(3).to.contain('baz');
}, "expected 3 to contain 'baz'");
// => Error: expected 'expected 3 to contain \'baz\'' to equal 'Expected an error'

This is indexOf bug.

indexOf(0, 'baz'); //=> 0

It fixed.