Add assertion to polymorphic_routes_test.rb

The assertion will ensure that the behavior doesn't regress.
  assert_equal "/projects", polymorphic_path("projects")

Remove FIXME related to polymorphic_url behavior.
polymorphic_url with Symbol or String works equally.
Example:

  default_url_options[:host] = "example.com"

  polymorphic_url(:projects)  # => "http://example.com/projects"
  polymorphic_url("projects") # => "http://example.com/projects"

Related to 37d4415a

Add missing module namespacing wrapper

Unfreeze interpolated string because it's unnecessary.

Remove deprecate passing string to `:if` and `:unless` conditional options [ci skip]

Follow up to #27608

Change engines guide to demonstrate maintained forum

Remove `:doc:` for `NumericalityValidator` [ci skip]

Change Rails engine guide to references a maintained project
Thredded instead of the abandoned project Forem. I chose
Thredded as Forem's closing note (below) suggests the choice.
https://github.com/rubysherpas/forem/blob/rails4/README.md

The `:doc:` was added in cdb9d7f4 but `NumericalityValidator` is already
`:nodoc:` class. `:doc:` is unneeded.

https://github.com/rails/rails/blob/master/activemodel/lib/active_model/validations/numericality.rb#L3

Fix typo in I18n Guide [ci skip]

Fix doc in Multibyte::Chars [ci skip]

Change ActiveModel::Validation to ActiveModel::Validations in comments

Remove redundant namespaces from sample code of `deprecated_method_wa…

Fix a tests of AR::ValueTooLong when using OracleAdapter

Indicate units of 'limit' in 'Integer' error message.

refs: #28011

Fix generator command for nested (namespaced) rails engine (take 2)

Remove unused DdlHelper in ForeignKeyTest

Use ActiveRecord `bigint` type, not SQL literal bigint

Oracle database itself does not have `bigint` SQL type, then it gets `ORA-00902: invalid datatype`.

It can be addressed by using ActiveRecord `bigint` type
because Oracle enhanced adapter recognizes ActiveRecord `bigint` type
and transfer it to its equivalent SQL type `NUMBER(19)`.

Should keep the primary key column order

Fixes CI failure caused by #27961.

https://travis-ci.org/rails/rails/jobs/201472146#L2390-L2409

Not ants were harmed! 🐜 [ci skip]

Simplify and optimize Postgres query for primary_keys()

Fix the return of `deprecate_methods` in doc [ci skip]

Freeze fragment cache related instrument name.