Move stray concern for nilifying blanks

* Was in `lib`, can now live in `Concerns::NilifyBlanks`
* actual genuine concern that autoloads, not the sort-of-concerns we previously moved, so needs to be in a fixed `Concerns` namespace.
In a console, as expected:

```ruby
2.1.2p95 :004 > include ActiveRecord::Concerns::NilifyBlanks
NameError: uninitialized constant ActiveRecord::Concerns
	from (irb):4
	from /Users/russ/.rvm/gems/ruby-2.1.2/gems/railties-4.0.8/lib/rails/commands/console.rb:90:in `start'
	from /Users/russ/.rvm/gems/ruby-2.1.2/gems/railties-4.0.8/lib/rails/commands/console.rb:9:in `start'
	from /Users/russ/.rvm/gems/ruby-2.1.2/gems/railties-4.0.8/lib/rails/commands.rb:62:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'
2.1.2p95 :005 > include Concerns::NilifyBlanks
 => Object
```

I still can't understand how the build passed, but don't want to waste time finding out. I've rebased it out anyway.