Check __proto__ instead of .prototype in assert.deepEqual()

Fixes joyent/node#4523, but that might not be the right thing to do here.

A [test case](https://github.com/joyent/node/blob/master/test/simple/test-assert.js#L115) for the assert module depends on the [original bug](https://github.com/joyent/node/issues/4523). I remove it with this patch, but perhaps removing the prototype check altogether and leaving the test case and bug as they currently stand is a good idea. The docs do say the assert API is locked.