use Array() instead of flatten

* move ActiveModel::Errors tests to errors_test.rb
* add spec coverage for add_on_empty and add_on_blank

Fix some indenting for some properties and closing }, remove tabs, and
merge two pre/code definitions.

[ci skip]

Current stable guides show the "Guides Index" links without underline,
just showing it when it's hovered. This restores the same functionality
for edge guides.

Also remove annoying pointer cursor while accessing guides index,
restoring previously functionality: now the pointer cursor only appears
when hovering a link.

Closes #8107 [ci skip]

ArgumentError is better suited than RuntimeError for this.

Use `tag!` instead of `method_missing` in `to_xml` conversions.

Since version `3.0.x` `Builder` caches method passed to `method_missing` each time. This commit replaces `method_missing` call with `tag!` call to prevent method redefinition on each `to_xml` call with the same builder.

Also refactor the test a bit.

There is no need to expose this accessor since we already have the query
method #permitted? that should handle this purpose.

message

Support :multiple option on input tags that also have :index

When you have an explicit index set, then when you build an input tag
with :multiple => true, it doesn't add [] to the end of its name, although
it should.

This comment is not valid since that `if` is there to make possible to
do:

    remove_index :users, :name

Instead of:

    remove_index :users, column: :name

What is a valid use case.

[ci skip]

This was there due historical reasons since
7dc45818 to give the user the
possibility to create unique indexes passing "UNIQUE" as the third
argument

Closes #8104

Fixed grammar in migration pending error.

Conflicts:
	actionpack/lib/action_controller/metal/mime_responds.rb
	activerecord/lib/active_record/attribute_methods.rb
	guides/source/working_with_javascript_in_rails.md

This reverts commit 3b890520.

Reason: changes made to a test, albeit in commented lines - non-uniform
change.

[ci skip]

This reverts commit cd17ee55.

Reason: Let's note that this is discouraged, not remove the docs

[ci skip]

This reverts commit f4ad0ebe, reversing
changes made to 8b2cbb3a.

Conflicts:
	actionpack/CHANGELOG.md

REASON: This added introduced a bug when you have a shorthand route
inside a nested namespace.

See
https://github.com/rafaelfranca/rails/commit/281367eb770faf8077c1fd6194188e92ed1637a1

Fixed mapping of Solomon Islands.