Your isArray implementation and IE6

One of the major selling-points of Expect.js for me, is that it's compatible with older browsers that I need to run my unit tests in.

So, just a heads-up: your implementation of isArray(), used in several places in the codebase, _will not work_ with foreign-context arrays in IE6 and IE7. You should either replace it with a simple `instanceof`, or if you truly want to support foreign-context arrays (please do, I use these. :), try [something else](http://whattheheadsaid.com/2010/10/cross-context-isarray-and-internet-explorer).

(Offending lines: [expect.js#746](https://github.com/LearnBoost/expect.js/blob/5a1e44c36ad14ee699bbe4e230564cfee176a1e4/expect.js#L746), [expect.js#1060](https://github.com/LearnBoost/expect.js/blob/5a1e44c36ad14ee699bbe4e230564cfee176a1e4/expect.js#L1060))