Fix for calling #flatten on an array of links

Calling #flatten on an array of links will attempt to call #to_ary on `resource`. This fixes it by explicitly defining `Link#to_ary` to return `nil`. Here are related issues on other projects talking about this issue:

https://github.com/rspec/rspec-mocks/issues/31
https://github.com/bundler/bundler/pull/1274