[Testing guide] Use "Fixtures API" instead of "fixture api".
[Testing guide] Using "Fixtures API" consistently

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.
Fix grammar on ActiveRecord::AttributeMethods doc.

[ci skip]
Remove custom errors page section from the guides

This pattern is too problematic and introduces a lot of edge cases:

  * On 4.2, the issue https://github.com/rails/rails/issues/15124 is
    back again.
  * needs to define each action for each http status otherwise the
    router raises ActionController::RoutingError (No route matches).
  * If the router has `match "/*username",...` and some action is
    missing, Rails will pick up the "match" and try to do its job.
  * encourages people to copy & paste programming. Not DRY.

[ci skip]
Merge branch 'master' of github.com:rails/rails
Add docs for AS::TimeWithZone + and -

The example was taken from the commit message 676d6a6.

[ci skip]
Merge branch 'document-time-with-zone-plus-and-minus'
Better doc for AS::DateTime#seconds_since_midnight

Adds examples and keeps coherent with the documentation of the
similar method `seconds_until_end_of_day`. [ci skip]
[ci skip] Fix output of video_tag helper
Don't error when `attributes` is called on a frozen AR model

`freeze` will ultimately end up freezing the `AttributeSet`, which in
turn freezes its `@attributes` hash. However, we actually insert a
special object to lazily instantiate the values of the hash on demand.
When it does need to actually instantiate all of them for iteration (the
only case is `ActiveRecord::Base#attributes`, which calls
`AttributeSet#to_h`), it will set an instance variable as a performance
optimization

Since it's just an optimization for subsequent calls, and that method
being called at all is a very uncommon case, we can just leave the ivar
alone if we're frozen, as opposed to coming up with some overly
complicated mechanism for freezing which allows us to continue to modify
ourselves.

Fixes #17960
Add foreign_type option for polymorphic has_one and has_many.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.
Remove unused "Developer" fixtures from tests

The `RecursiveCallbackDeveloper` and `ImmutableMethodDeveloper` classes
are not used anymore in tests, and neither is the `@cancelled` variable.
Allow custom handling of non-standard types in `time_zone_conversion`

PostgreSQL for example, allows infinity as a valid value for date time
columns. The PG type has explicit handling for that case. However, time
zone conversion will end up trampling that handling. Unfortunately, we
can't call super and then convert time zones.

However, if we get back nil from `.in_time_zone`, it's something we
didn't expect so we can let the superclass handle it.

Fixes #17971
Update test case for TZ aware attributes

The test added in 42418cfc94d1356d35d28d786f63e7fab9406ad6 wasn't
actually testing anything, since the bug was with TZ aware attributes
only.
[ci skip] Fix typo
Merge pull request #17977 from yui-knk/fix/typo

[ci skip] Fix typo
Merge pull request #17972 from claudiob/remove-unused-callback-fixtures

Remove unused "Developer" fixtures from tests
Unused csv fixture file
Fix options overwritten by super
Merge pull request #17975 from merongivian/add_test_for_search_field

Fix options overwritten by super