export: false option is not working correctly

Hi,

I found the export: false option on defcall does not actually work, and the tests for it do not detect this because they are testing for a higher arity. What happens is a function is created that will try to send messages to a server named "false", so it has a lower arity than if it had no export statement at all. The changes I made to the gen_server_test.exs will demonstrate the issue. 

I included a fix - I'm not sure its how you would want to do it but there it is if its helpful.

Thanks, Jeremy