Fix infinite recursion

The method for testing whether to include an association was causing
an infinite loop when two models referenced each other.

test/adapter/json_api/belongs_to_test.rb

@@ -7,6 +7,7 @@ module ActiveModel
         class BelongsToTest < Minitest::Test
           def setup
             @author = Author.new(id: 1, name: 'Steve K.')
+            @author.bio = nil
             @post = Post.new(id: 42, title: 'New Post', body: 'Body')
             @anonymous_post = Post.new(id: 43, title: 'Hello!!', body: 'Hello, world!!')
             @comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')