Include an assertNotContains for responses

You could already do this with assertContains(count=0), but I don't have any particular objection otherwise.

BUT

Needs tests and documentation. Remember people, a patch is almost worthless if it doesn't prove to me that it works, and tells me how to use it.

Assertion with test and documentation.

There it is with test and documentation. I wanted to check that the code was acceptable before putting more time on it. If you need more tests or a different documentation, let me know and I'll try to do it.

I actually haven't thought about count=0 because I almost never use count, but even then I think using NotContains?() is more readable than Contains(count=0), and tests can always use some improvements in readability. Also I was thinking if maybe this has to be thin wrapper to Contains(count=0), but then, the error message would not be as informative. What do you think?

(In [7578]) Fixed #7165 -- Added an assertNotContains() method to the test client. Thanks for the suggestion and implementation, J. Pablo Fernandez <pupeno@pupeno.com>.