feat(ngMock): allow the use of a predicate to match data

When using $httpBackend, the data in definitions and expectations can only
be defined as a string (which must be matched exactly) or as a regexp.
Both are not very practical when it comes to matching JSON objects. This
commits adds the possibility to pass a predicate function, which should
return a truthy value if the data matches with the expectation and a falsy
value if it doesn't.