Improve stringification of captor.capture() matcher

[searls]

right now, captor.capture() matchers are printed out with an ugly toString() in error messages. they should be niceified like the other matchers.

currently just got:

All calls of the test double, in order were:
  - called with `("target", undefined, function (name, originalValue) { 
      return (0, _chainPrototype2.default)(overwriteChild(originalValue, '.' + name), name, originalValue, original); 
    })`.                                                                                    

That whole third arg should have been captor.capture() or the like