In response to https://github.com/rails/rails/issues/32917

In the current implementation, ActiveStorage passes all options to the underlying processor,
including when a key has a value of false.

For example, passing:

```
avatar.variant(resize: "100x100", monochrome: false, flip: "-90")
```

will return a monochrome image (or an error, pending on ImageMagick configuration) because
it passes `-monochrome false` to the command (but the command line does not allow disabling
flags this way, as usually a user would omit the flag entirely to disable that feature).

This fix only passes those keys forward to the underlying processor if the value responds to
`present?`. In practice, this means that `false` or `nil` will be filtered out before going
to the processor.

One possible use case would be for a user to be able to apply different filters to an avatar.
The code might look something like:

```
  variant_options = {
    monochrome: params[:monochrome],
    resize:     params[:resize]
  }

  avatar.variant(*variant_options)
```

Obviously some sanitization may be beneficial in a real-world scenario, but this type of
configuration object could be used in many other places as well.

- Add removing falsy values from varaints to changelog

- The entirety of #image_processing_transformation inject block was wrapped in `list.tap`
 to guard against the default `nil` being returned if no conditional was called.

- add test for explicitly true variant options

Follow up of #32418.

Finalize transaction record state after real transaction

After a real (non-savepoint) transaction has committed or rolled back,
the original persistence-related state for all records modified in that
transaction is discarded or restored, respectively.

When the model has transactional callbacks, this happens synchronously
in the `committed!` or `rolled_back!` methods; otherwise, it happens
lazily the next time the record's persistence-related state is accessed.

The synchronous code path always finalizes the state of the record, but
the lazy code path only pops one "level" from the transaction counter,
assuming it will always reach zero immediately after a real transaction.
As the test cases included here demonstrate, that isn't always the case.

By using the same logic as the synchronous code path, we ensure that the
record's state is always updated after a real transaction has finished.

Don't generate yarn's contents in `app:update` task if it's skipped

Fix documentation for ActionController::Params#fetch

Fixes #32928.

Make it clear that the return value is converted to an
instance of ActionController::Parameters if possible

Generate ActiveStorage attachment getter and setter methods in mixin

Generated attachment getter and setter methods are created within
the model's `GeneratedAssociationMethods` module to allow overriding
and composition using `super`.

Includes tests for new functionality.
Co-authored-by: NJosh Susser <josh@hasmanythrough.com>
Co-authored-by: NJamon Douglas <terrildouglas@gmail.com>

Fix user_input_in_time_zone to coerce non valid string into nil

Active storage: Image variant options not correct in docs

[ci skip]

### Steps to reproduce

Using Rails 5.2.0

When following this example:

http://api.rubyonrails.org/classes/ActiveStorage/Variant.html

`avatar.variant(resize: "100x100", monochrome: true, flip: "-90")`

### Expected behavior

Image should be rendered as flipped.

### Actual behavior

I get an error:

> failed with error: gm mogrify: Unrecognized option (-90).

### Fix:

According to: https://github.com/minimagick/minimagick the option should be called rotate:

`avatar.variant(resize: "100x100", monochrome: true, rotate: "-90")`

So **flip** changed to **rotate**.

### System configuration
**Rails version**: 5.2.0

**Ruby version**: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]

[David Robertson & George Claghorn]

Before it was coercing an invalid string into "2000-01-01 00:00:00".

Inside user_input_in_time_zone we call in_time_zone on the value and value can be a String.

Originally `SingularAssociation#replace` abstract method is private, and
doesn't intend to be called directly.

Follow up of #19171 and #26825.

Add math test

After #449 was merged math can be done on these
nodes, adding a test file to unit test all the
math operators.

Don't generate `config/spring.rb` in `app:update` task when spring isn't loaded

Add available transformations to docs

`foreign_key`, `json` and `virtual` are also available.

Follow up of #32605.

Remove ActiveRecord::Transactions#rollback_active_record_state!

Don't clear transaction state after manual rollback

`rollback_active_record_state!` was removed from `save!` but not `save`
in da840d13. I believe that leaving it
in `save` was a mistake, since that commit was intended to move the
rollback logic from the `save`/`save!` call to the transaction stack.

As of 67d8bb96 the record's original
state is lazily restored the first time it's accessed after the
transaction, instead of when a rollback occurs. This means that the call
to `restore_transaction_record_state` here has no effect: the record's
transaction level is incremented twice (in rollback_active_record_state!
and `with_transaction_returning_status`), isn't decremented again until
the the `ensure` block runs, and won't hit zero until the next time
`sync_with_transaction_state` is called.

I'm not sure what originally wanted to pass to the argument.
However, as long as see the document added along with the commit, it seems just
to be mistaken that trying to pass the error instance.
https://github.com/rails/rails/pull/30622/files#diff-59beb0189c8c6bc862edf7fdb84ff5a7R64

Fixes #32853

Remove unused `remove_file` method

Remove reference to old `:text` rendering option