Fix attr_accessible handling

[ahacking]

Closure Tree now works for me on Rails4 with attr_accessible

The class_eval used for building the new hierarchy class was not working as intended.

The accessible_attributes.present? check always returns false when building the new hierarchy class since the whitelist is always empty.

[mceachen]

The test is failing with ruby 1.9.3:

/home/travis/build/mceachen/closure_tree/lib/closure_tree/support.rb:31:in `hierarchy_class_for_model': uninitialized constant Activodel::ForbiddenAttributesProtection (NameError)
    from /home/travis/build/mceachen/closure_tree/lib/closure_tree/support.rb:30:in `class_eval'
    from /home/travis/build/mceachen/closure_tree/lib/closure_tree/support.rb:30:in `hierarchy_class_for_model'

Note the weird "Activodel"

[ahacking]

Thats a strange error for sure, it worked for me, ruby -v reports:
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin10.8.0]

Closure tree doesn't work with attr_accessible and AR 4.0.0 as it stands currently, can you just apply the changes in support_flags.rb or do you need another PR?

[mceachen]

I partially cherry-picked out support_flags in ff8331e and added some documentation in a1d8076. I can push a new version once Travis goes green.

[ahacking]

Sounds great. Thanks!

[rubiii]

@mceachen just wanted to report that the change you cherry-picked seems to work fine.

[mceachen]

This got pushed into v4.2.8