fix: default message for test with object

[vonagam]

Two things:

1. Fix. If you use test() with multiply arguments like test('name', func) then you will be provided with default message (locale.default). But not if you use it with opts object without message test(opts) or with second argument as undefined test('name', undefined, func) (it is assumed right now that since there are 3 arguments then message is present). Now all variants treated equally and if message is undefined then it will be set to locale.default.

2. Small feature. Now it is possible to write test(func). Since name is needed only for exclusive tests it can be omitted (like it is possible to omit it in opts form).