Ensure null objects behave correctly with to_a and to_ary

[JonRowe]

Further to #357 we could return nil when the double is a as_null_object, we shouldn't change the behaviour when it's not a null object but this could be considered slightly more correct behaviour.

[coveralls]

Changes Unknown when pulling f7caa59 on revisited_bugfix_for_doubles_in_arrays into * on master*.

[alindeman]

Need any help with 1.8.7 here @JonRowe? I have some bandwidth if needed.

[JonRowe]

Thanks @alindeman I sussed it, 1.8.7 has a default implementation of to_a which is used regardless of your intent with method_missing, was just not transparent to me last night that what was happening

[alindeman]

Would this look any better as a case statement?

case message
when :to_int
  0
when :to_a, :to_ary
  nil
end

I'm not sure. What do you think?

[coveralls]

Changes Unknown when pulling 3efca73 on revisited_bugfix_for_doubles_in_arrays into * on master*.

[coveralls]

Changes Unknown when pulling faa04cf on revisited_bugfix_for_doubles_in_arrays into * on master*.

[coveralls]

Changes Unknown when pulling 71f39f7 on revisited_bugfix_for_doubles_in_arrays into * on master*.

[alindeman]

Looks good to me.