added check for logging negative zero

As @Lexicality mentions in issue #223, when asserting ```0``` against ```-0```, Chai throws ```AssertionError: expected 0 to deeply equal 0``` without any further information.

With the check in this PR, Chai throws ```AssertionError: expected 0 to deeply equal -0```.
I totally agree with that! Thanks for pointing it out, @keithamus ! :smile: