Fix custom site for oauth

Hi, I've been using this Github client library and found it easy to work with. However, I've encountered this problem with authorization. The problem is demostrated by commit e201c7e and fixed in the later commit.

I admit that I'm breaking the current design a little bit. But I guess the `Github::Client#client` method should be private anyway and I believe my approach actually made it harder to misuse.

Feel free to tell me what more to do to get this patch merged so that I can remove some workaround from my app :)