I found out a strange saving issue with many-to-many association 

Hello, I found out a strange saving issue with many-to-many association. below is the example:

```ruby
class Person < ActiveRecord::Base
  has_many :references
  has_many :jobs, through: references
end

class Reference < ActiveRecord::Base
  belongs_to :person
  belongs_to :job
end

class Job < ActiveRecord::Base
  has_many :references
  has_many :person, through: references
end
```

When i new a record then save, it doesn't save data in references table.

```ruby
@job = @person.jobs.new
@job.save

@job.persisted? # will return true
@person.jobs.reload # will return empty
```

If i change the association name to singular in Job model

```ruby
class Job < ActiveRecord::Base
  has_many :reference
  has_many :person, through: reference
end
```

It works. I write a test for this situation. please check it, thanks.