Allow missing operators on error objects.

[supertest](https://github.com/visionmedia/supertest/) kicks out an error object that lacks an operator property (see [code](https://github.com/visionmedia/supertest/blob/master/lib/test.js#L207-L213)). Most of the time this has no impact, but when there is also a multi-line object to display as actual or expected value the operator must be formatted. The indent function must therefore be undefined-safe.