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 :persons, 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, it works.

```ruby
class Job < ActiveRecord::Base
  has_many :reference
  has_many :persons, through: :reference
end
```

I original want to fix it, but activerecord is too complex to me.

There's a test for this situation, check it please