Spread parameters won't be expanded

[falsandtru]

In the following case, o spread parameter should be expanded in the assertion message.

        it('Object Rest/Spread', () => {
            expectPowerAssertMessage (() => {
                var o = { a: 1, b: 2 };
                assert.deepStrictEqual({ ...o, c: 5 }, { a: 1, b: 2, c: 3 });
            }, [
                '  assert.deepStrictEqual({ ...o, c: 5 }, { a: 1, b: 2, c: 3 })',
                '                         |               |                    ',
                '                         |               Object{a:1,b:2,c:3}  ',
                '                         Object{a:1,b:2,c:5}                  '
            ]);
        });

https://github.com/power-assert-js/power-assert/blob/master/test/tobe_instrumented/assertion.es20xx.js#L78

Oddly, it is realized only with karma-espower-preprocessor.

https://travis-ci.org/twada/power-assert-keeper/jobs/406328262#L988

[falsandtru]

ping @twada