Fixed a bug in AR::Base#respond_to?

Before:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  NoMethodError: undefined method `abstract_class?' for Object:Class

After:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  => false
`scope` now raises on "dangerous" name conflicts

Similar to dangerous attribute methods, a scope name conflict is
dangerous if it conflicts with an existing class method defined within
`ActiveRecord::Base` but not its ancestors.

See also #13389.

*Godfrey Chan*, *Philippe Creux*
`enum` now raises on "dangerous" name conflicts

Dangerous name conflicts includes instance or class method conflicts
with methods defined within `ActiveRecord::Base` but not its ancestors,
as well as conflicts with methods generated by other enums on the same
class.

Fixes #13389.
Merge pull request #13450 from chancancode/ar_raise_on_conflict

Raise an error if `scope` or `enum` is about to add a conflicting method to the class

Fixed #13389
Mention find_each in find_in_batches doc [ci skip]
Merge pull request #13877 from marcandre/find_in_batches

Mention find_each in find_in_batches doc [ci skip]
Merge pull request #13201 from marcandre/find_in_batch_enumerator

`find_in_batches` now returns an `Enumerator`

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/relation/batches.rb
Enhance errors while retrieving database config

Right now if there is an error retrieving database configuration the intent of the error (what the code was trying to do while you got the error) could be more explicit. 

Instead of this error:

```
Invalid DATABASE_URL: nil
(erb):9:in `rescue in <main>'
(erb):6:in `<main>'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `eval'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `result'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application/configuration.rb:98:in `database_configuration'
/Users/schneems/Documents/projects/rails/activerecord/lib/active_record/railtie.rb:41:in `block in <class:Railtie>'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `instance_exec'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `each'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application.rb:339:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/engine/railties.rb:13:in `each'
```

I propose we issue this error:

```
Cannot load `Rails.application.database_configuration`:
Invalid DATABASE_URL: nil
(erb):9:in `rescue in <main>'
(erb):6:in `<main>'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `eval'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `result'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application/configuration.rb:98:in `database_configuration'
/Users/schneems/Documents/projects/rails/activerecord/lib/active_record/railtie.rb:41:in `block in <class:Railtie>'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `instance_exec'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `each'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application.rb:339:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/engine/railties.rb:13:in `each'
```
Added some style changes in asset pipeline documentation
Fix documentation of new controller filters api [ci skip]

The api for filters with classes change and the guides weren't updated.
Now the class must respond for methods with the same name as the filter,
so the `before_action` calls a `before` method, and so on.

The method `#filter` has been deprecated in 4.0.0 and has been removed
in 4.1.0: #7560
Merge pull request #13871 from kassio/fix-filters-with-classes-documentation

Fix documentation of filters api with classes [ci skip]
Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861
Merge pull request #13876 from sowjumn/master

Added some style changes in asset pipeline documentation
Merge pull request #13867 from mauricio/bug-13861

Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861
Aesthetic
Merge pull request #13879 from schneems/schneems/more-explicit-database-config-error

Enhance errors while retrieving database config
Implement a simple stub feature to use in the Time travel helpers
Alias the original method first to avoid warnings
Use instance method instead of before hook
Change the class and method visibility