Fixes swapping objects.

[jamesplease]

This resolves #30.

It needs tests, but I figured I'd open the PR to get the discussion going.

To test,

// On master, you'll get an error
var patch = jsonpatch.compare({a: {}}, {a: null});

// On my branch, you get a diff:
patch === {
  op: "replace"
  path: "/a"
  value: true
};

[jamesplease]

ping @warpech @Starcounter-Jack @maca

[ifeltsweet]

Why is this not being merged?

[NiotoMOto]

@Starcounter-Jack @tomalec can you merge this please ?

[warpech]

I will merge it in 0.5.1

[gdibble]

good fix!