Fix migrations with enable_extension

When using ActiveRecord::Base.table_name_prefix and/or
table_name_suffix, extension names got the same treatment as table names
when running migrations. This led to migrations that tried to call, for
example, enable_extension("prefix_hstore") on the connection.
Hash#deep_*_keys(!) recurse into nested arrays.

Following methods now recursively transform nested arrays, too.

* Hash#deep_transform_keys
* Hash#deep_transform_keys!
* Hash#deep_stringify_keys
* Hash#deep_stringify_keys!
* Hash#deep_symbolize_keys
* Hash#deep_symbolize_keys!
Check authentication scheme in Basic auth

`authenticate_with_http_basic` and its families should check the authentication
schema is "Basic".

Different schema, such as OAuth2 Bearer should be rejected by basic auth, but
it was passing as the test shows.

This fixes #10257.
Run login_procedure only when the auth_scheme is valid
Refactor AR's validations_test.rb

The `:context` switch feature is implemented in
ActiveRecord::Validations#valid? method, so we should rename the test
names, and execute `valid?` in the test.

Change test name in AR's validations_test.rb

This test is testing save method's code
Fix serialized field returning serialized data after update_column
Fixes Issue #13466.

Changed the call to a scope block to be evaluated with instance_eval.
The result is that ScopeRegistry can use the actual class instead of base_class when
caching scopes so queries made by classes with a common ancestor won't leak scopes.
Fix Formatting in AR Readme

Don't make free form text appear in code blocks [ci skip]
[ci skip] document type_cast_for_write
[ci skip] document ActionDispatch::HTTP::Headers
Allow custom asset host to be passed in asset_url
set a constant for the "anonymous" habtm model fixes #15022

I wanted the middle model to be anonymous, but we have to give it a name
for marshal support. :'(
add more tests around the polymorphic url api
copy edits [ci skip]
fix formatting errors [ci skip]
Improve documentation for asset_url

Updated CHANGELOG.md with entry about :host in asset_url
[ci skip] Minor improvements on routing.md.
Merge pull request #15029 from JuanitoFatas/docpatch/routing

[ci skip] Minor improvements on routing.md.
Change 'a' to 'an' [ci skip]
Merge pull request #15030 from mechanicles/a-to-an

Change 'a' to 'an' [ci skip]