Fix #2109: stringify should handle objects with non-coercible length

[joshlory]

Fix for #2109. When comparing objects with certain length properties, the reporter throws when stringifying the diff:

it('compare two objects', function () {
   assert.deepEqual({length: {toString: 0}}, {length: 1});
});

Root cause is that some JS objects cannot be coerced to a Number:

> Number({toString: 0});
TypeError: Cannot convert object to primitive value

> Number({a: 0});
< NaN

[danielstjules]

Looks good :) Can you fix the conflicts?

[joshlory]

Rebased!

[joshlory]

@danielstjules is this PR ready to merge? Thanks!

[danielstjules]

Thanks again!