eql does not work if an array item is undefined in IE8

The following test fails

``` js
describe('eql', function () {
it('should not remove undefined', function () {
var arr = []
arr.push('1')
arr.push('2')
arr.push(undefined)
expect(arr).to.eql(['1', '2', undefined])
})
})
```

This is because the `keys` function used [here](https://github.com/Automattic/expect.js/blob/master/index.js#L941) will not iterate over an undefined array item in IE8 **unless the value was pushed to the array**.

If on line 941 the keys function is called on a duplicate of the array, this could be resolved.

``` js
try{
var ka, kb, key, i;
if (isArray(a) && isArray(b)) {
ka = keys(cloneArray(a));
kb = keys(cloneArray(b));
} else {
ka = keys(a);
kb = keys(b);
}
} catch (e) {//happens when one is a string literal and the other isn't
return false;
}

function cloneArray (arr) {
var clone = [];
for (var i = 0; i < arr.length; i++) {
clone.push(arr[i]);
}
return clone;
}

function isArray (obj) {
return Object.prototype.toString.call(obj) === '[object Array]';
}
```

Start at index.js around line 941 and reproduce the eql test shown in the issue in IE8, focusing on how keys handles arrays containing a pushed undefined value. Done means the test passes and eql preserves that undefined array item without changing the existing non-array behavior.

Written by the indexing model from the issue text.