Make the apply_inflections method case-sensitive

Basic support for adding and removing foreign keys

The name of the foreign key is not relevant from a users perspective.
Using random names resolves the urge to rename the foreign key when the
respective table or column is renamed.

This allows to create and remove foreign keys without specifying a column.

respect `table_name_prefix` and `table_name_suffix`.

These methods shouldn't be added to `SecureRandom`, as they are neither secure nor random. The more appropriate place for this seems to be `Digest`, so we should move them there. (Pull request welcomed!)

Marking this `:nodoc:` for now, so we don't accidentally ship it as public API.

See https://github.com/rails/rails/pull/15306/files#r13055862 for details.

Move writing unknown column exception to null attribute

Consolidate testing of update_all type casting

Making this change revealed several subtle bugs related to models with
no primary key, and anonymous classes. These have been fixed as well,
with regression tests added.

We have several test cases on "tricky" types that are essentially
testing that `update_all` goes through the same type casting behavior as
a normal assignment + save. We recently had another case to add this
test for another type in https://github.com/rails/rails/pull/12742.

Rather than testing this separately for every type which is "tricky"
when round tripping, let's instead have a fairly exhaustive test that
ensures we're getting the correct values at every step for `update_all`.
Given the structure of the code now, we can be confident that if the
type is correct, and `update_all` is type casting correctly, we're going
to get the right behavior for all types.

Deprecate automatic counter caches on has_many :through

Reliant on https://github.com/rails/rails/pull/15747 but pulled to a
separate PR to reduce noise. `has_many :through` associations have the
undocumented behavior of automatically detecting counter caches.
However, the way in which it does so is inconsistent with counter caches
everywhere else, and doesn't actually work consistently.

As with normal `has_many` associations, the user should specify the
counter cache on the `belongs_to`, if they'd like it updated.

`Attribute` should know about its name

This allows using polymorphism for the uninitialized attributes raising
an exception behavior.

Make the isolated tests run on JRuby

Since d3071db1, the apply_inflections method check if the downcased
version of a string is contained inside the "whitelist" of uncountable
words. However, if the word is composed of capital letters, it won't be
matched in the list while it should.

We can't simply revert to the previous behavior as there is a
performance concern (benchmarked over /usr/share/dict/words):

Before d3071db1  135.610000   0.290000  135.900000 (137.807081)
Since d3071db1   22.170000    0.020000  22.190000  ( 22.530005)
With the patch   22.060000    0.020000  22.080000  ( 22.125771)

Benchmarked with http://git.io/aFnWig

This way, the solution is to put the down-case version of words inside
the @uncountables array.

Allow proc and symbol as values for `only_integer` of `NumericalityValidator`

Closes #15919

This will make it less painful to add additional properties, which
should persist across writes, such as `name`.

Conflicts:
	activerecord/lib/active_record/attribute_set.rb

Move behavior of `read_attribute` to `AttributeSet`

Conflicts:
	activerecord/lib/active_record/attribute_set.rb
	activerecord/test/cases/attribute_set_test.rb

Encapsulate knowledge of type objects on `ActiveRecord::Result`

Move `attributes_before_type_cast` to `AttributeSet`

Conflicts:
	activerecord/lib/active_record/attribute_set.rb
	activerecord/test/cases/attribute_set_test.rb