Resolve warnings by checking if @attributes is defined

[bcardarella]

When @attributes is undefined (in the case of some of the tests using .allocate instead of .new on an AR class) a warning will be thrown.

As this adds an extra conditional it could be a performance hit but I suspect the only time anybody is really calling #inspect on an AR object is from within the console so it shouldn't be a big deal.

[josevalim]

If someone is calling allocate, they should call initialize themselves. It is not worth adding this check because of thi allocate special case. I would fix this instead by calling model.instance_variable_set in the test itself.

[bcardarella]

Ok, I'll update the working branch and resubmit