Fix AR#method_missing re-dispatching into overwritten attribute methods.

This was happening when a `super` call in an overwritten attribute method
was triggering a method_missing fallback, because attribute methods
haven't been generated yet.

  class Topic < ActiveRecord::Base
    def title
      # `super` would re-invoke this method if define_attribute_methods
      # hasn't been called yet resulting in double '!' appending
      super + '!'
    end
  end
Install Spring preloader when generating new applications
Add nodoc to added VariantFilter class
Improve a couple exception messages related to variants and mime types

Avoid one-liner conditionals when they are too big. Avoid concatenating
strings to build error messages. Improve messages a bit.
Merge pull request #13109 from chancancode/json_guides

Added JSON related items to the 4.1 release notes
minor 4_1_release_notes changes. [ci skip]
Merge pull request #12958 from jonleighton/spring

Install Spring preloader when generating new applications
add Spring as major feature in the 4.1 release notes. [ci skip]

Follow up to #12958.
remove variants paragraph indent in release notes. [ci skip]
Fix documentation of number_to_currency helper

Now users have to explicit mark the unit as safe if they trust it.

Closes #13161

Conflicts:
	actionpack/lib/action_view/helpers/number_helper.rb
	actionpack/test/template/number_helper_i18n_test.rb
Merge pull request #12403 from thedarkone/attr-method-missing-fix

Fix AR#method_missing re-dispatching into overwritten attribute methods

Conflicts:
	activerecord/lib/active_record/attribute_methods.rb
Fix issue where TextHelper#simple_format was calling missing 'raw' method
Merge pull request #13152 from mariovisic/text_helper_missing_raw_method

TextHelper#simple_format attempts to call undefined raw method
Formatting, capitalization, and punctuation fixes [ci skip]

Second try on this commit.
Merge pull request #13167 from fluxusfrequency/patch-3

Formatting, capitalization, and punctuation fixes [ci skip]
mention Active Record enums as major feature for 4.1. [ci skip]
remove nil in NullType#ref.

Return Nil is implicit in a method and this syntax is used in the others
classes
Merge pull request #13170 from acapilleri/remove_return_nil

remove nil in NullType#ref.
removed duplicate test case
Merge pull request #13171 from kuldeepaggarwal/test-case-updation

removed duplicate test case