[ActiveStorage] Disable variant options when false or nil present

Add Enumerable#index_with.

In the app I'm working on I've wished that index_by had a buddy that would
assign the hash value instead of the key multiple times.

Enter index_with. Useful when building a hash from a static list of
symbols. Before you'd do:

```ruby
POST_ATTRIBUTES.map { |attr_name| [ attr_name, public_send(attr_name) ] }.to_h
```

But now that's a little clearer and faster with:

````ruby
POST_ATTRIBUTES.index_with { |attr_name| public_send(attr_name) }
```

It's also useful when you have an enumerable that should be converted to a hash,
but you don't want to muddle the code up with the overhead that it takes to create
that hash. So before, that's:

```ruby
WEEKDAYS.each_with_object(Hash.new) do |day, intervals|
  intervals[day] = [ Interval.all_day ]
end
```

And now it's just:

```ruby
WEEKDAYS.index_with([ Interval.all_day ])
```

It's also nice to quickly get a hash with either nil, [], or {} as the value.

Exception wording change

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

Follows the change from 6fac9bd5, so the naming is consistent.

Bump minimum SQLite version to 3.8

To prevent redundant `to_s` like https://github.com/rails/rails/pull/32923#discussion_r189460008
automatically in the future.

These OS versions have SQLite 3.8 or higher by default.

- macOS 10.10 (Yosemite) or higher
- Ubuntu 14.04 LTS or higher

Raising the minimum version of SQLite 3.8 introduces these changes:

- All of bundled adapters support `supports_multi_insert?`
- SQLite 3.8 always satisifies `supports_foreign_keys_in_create?` and `supports_partial_index?`
- sqlite adapter can support `alter_table` method for foreign key referenced tables by #32865
- Deprecated `supports_multi_insert?` method

So do not expose `PostgreSQLTypeMetadata` in the doc too.

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.