checkout "association_chain" method for default behavior

Hello, dudes! :rage2:
I have a little trouble with nested controllers and singleton option.

There are my models:

```ruby
class Speciality < ActiveRecord::Base
  has_many :subspecialities
end

class Subspeciality < ActiveRecord::Base
  belongs_to :speciality
  has_one :work_plan
end

class WorkPlan < ActiveRecord::Base
  belongs_to :subspeciality
end
```

and their controllers:

```ruby
class SpecialitiesController < ApplicationController
  inherit_resources
end

class SubspecialitiesController < ApplicationController
  inherit_resources
end

class WorkPlansController < ApplicationController
  inherit_resources

  defaults :singleton => true

  belongs_to :speciality do
    belongs_to :subspeciality
  end
end
```

When I visit specialities/1/subspecialities/2/work_plan/new I got an error:

```ruby
 undefined method `specialities' for #<Subspeciality:0x00000000000000>
```

I found on 138 line of https://github.com/josevalim/inherited_resources/blob/master/lib/inherited_resources/base_helpers.rb#L135-L147 symbols_for_association_chain reversed if singleton option is true.
Why was symbols_for_association_chain reversed? I can rewrite belongs_to in my controller in reverse order but why single option change default behavior?

PS

I checkouted the association_chain method to state before this commit 7f455095f55f1b9372ac28ce4e5cd8679b93a231 (https://github.com/josevalim/inherited_resources/commit/7f455095f55f1b9372ac28ce4e5cd8679b93a231#diff-9e560d176d0aba6271a143b4b0631e1d).
All tests are passed.