Explicitly check null and undefined values in exports.type

[chromakode]

Some older JS engines, such as PhantomJS, return the window object instead of "[object Null]", as would be expected here, crashing the test runner.

[boneskull]

@chromakode thanks, but where does the test runner crash? can you please add a test as well?

[chromakode]

Thanks for the review :) I believe the test runner crashes in the process of formatting object diff output. For a test like:

chai.assert.deepEqual({a: null}, {a: 'test'})

I get a relatively inscrutable output like:

TypeError: 'null' is not an object (evaluating 'value.toString')

With an unrelated traceback.

I considered writing a test case that shimmed Object.prototype.toString, but wasn't sure this would be a valid test. I'll add it and update this PR.

[chromakode]

Ok @boneskull , added a test to replicate the behavior of PhantomJS and rebased. I also added a test for type(undefined) in the general case, for consistency.

[jbnicolai]

Thanks for the work, @chromakode, and including a testcase!

[chromakode]

Great! Thanks for merging it. :)