Support Object.create(null) in JSON diffing

[danbeam]

Addresses #492

I was recently trying to use diff to compare output from a library that makes use of Object.create(null). This seems valid (if not encouraged) in some situations, but diff was throwing as the typeof this is 'object' and it is !== null but obj.hasOwnProperty does not exist.

Instead, use Object.prototype.hasOwnProperty.call(obj, key) similar to the way that Object.prototyope.toString is used elsewhere in this library.

[ExplodingCabbage]

Good catch; much appreciated!