Fix doc for `permitted_params`

`permitted_params` returns params that contains the resource name as its root key.

Such as:
``` ruby
  {
    'post' => {
      'title' => 'Rails is omakase.',
      'author' => 'DHH'
    }
  }
```
Merge pull request #3199 from tricknotes/fix-doc-for-permitted_params

Fix doc for `permitted_params`
Apply decorator to CSV export; fixes #3198
Convert specs to RSpec 2.99.0 syntax with Transpec

This conversion is done by Transpec 2.2.0 with the following command:
    transpec -f

* 102 conversions
    from: it { should ... }
      to: it { is_expected.to ... }

* 27 conversions
    from: be_true
      to: be_truthy

* 26 conversions
    from: == expected
      to: eq(expected)

* 13 conversions
    from: be_false
      to: be_falsey

* 8 conversions
    from: it { should_not ... }
      to: it { is_expected.not_to ... }

* 7 conversions
    from: pending
      to: skip

* 7 conversions
    from: pending 'is an example' { }
      to: skip 'is an example' { }

* 4 conversions
    from: describe 'some request' { }
      to: describe 'some request', :type => :request { }

* 2 conversions
    from: expect(obj).to have(n).sidebar_sections
      to: expect(obj.sidebar_sections.size).to eq(n)

* 1 conversion
    from: expect(obj).to have(n).menu_items
      to: expect(obj.menu_items.size).to eq(n)

See also: https://github.com/yujinakayama/transpec#supported-conversions
Upgrade to RSpec 3.0
Switch spec_helper to rails_helper, in accord with upgrade guidelines