• # tests contradicts each other

I would like to give some feedback about ...

HTTP_USER_AGENT:

```Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36
```

the third test, as below seems to require a None result being a valid result and let the output of f and g compare to each other.

```    assert checkio(lambda n:("Fizz "*(1-n%3) + "Buzz "*(1-n%5))[:-1] or str(n),
lambda n:('Fizz'*(n%3==0) + ' ' + 'Buzz'*(n%5==0)).strip())\
(7)==('7','different'), "fizz buzz, third"
```

While at the same time, the below test seems to imply that a None result should be treated as an error.

```f = lambda x: abs(x)
def g(x):
if x>0:
return x
elif x<0:
return -x
assert checkio(f,g)(0) == (0,'g_error')
```
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