remove reference to `country_options_for_select` and `country_select` from guide [ci skip]

these methods were removed in 2d27b82d

Create Adding images in Action Mailer Views on guides

[skip ci] #distinct instead of #uniq

[ci skip]

as #uniq will be removed from Rails 5.0 as per the Active Support
exception raised:

ActiveSupport::DeprecationException: DEPRECATION WARNING: uniq is
deprecated and will be removed from Rails 5.0 (use distinct instead).

changes names in guides to better reflect diversity

Not much of a thought leader if I can't spell it correctly 😉

After 908cfef6 was introduced fixtures that did not set an enum would
return nil instead of the default enum value.

The fixtures should assume the default if a different enum is not
defined.

The change checks first if the enum is defined in the fixture before
setting it based on the fixture.

[ci skip] Don't use TrueClass, FalseClass in docs

This sort of documentation style comes from 2009, probably due to
the merging of merb (see https://github.com/rails/rails/commit/38b608ecab2441cd0c4e75bc08bdf57fcf85dd71#diff-017d9bc9b1d2bdae199b938d72c15488R120).

Rails follows Ruby's convention to define which values are "truthy" or
"falsey", so there is no need to specify that the returned value must
strictly be a TrueClass or FalseClass. /cc @fxn

[ci skip] Improve the url_for documentation

Clarify the `url_for` usage in mailers.

Re-add the documentation about `url_for` and Route's path parameters,
first introduced by 5c4f1859.
This was reported on #15097 and until it is decided to deprecate it
or not, I believe the documentation should exist.

Only one constraint option can be used at a time (except for the minimum
and maximum ones that can eventually be combined). However, other
options can be used with them (e.g. the validation failure message).
So let's make the distinction between these two different options
categories.

[Yves Senn, Matthew Draper & Robin Dupret]

Avoid crashing when minitest-rails is loaded.

The improvments to the test runner's integration with minitest in commit
b6fc8e25 add methods to the Minitest
module that refer to the Rails module. Unfortunately, when the
minitest-rails gem is loaded, the reference is incorrectly resolved to
the Minitest::Rails module.

The changes introduced to through associations in c80487eb were quite
interesting. Changing `relation.merge!(scope)` to `relation =
relation.merge(scope)` should in theory never cause any changes in
behavior. The subtle breakage led to a surprising conclusion.

The old code wasn't doing anything! Since `merge!` calls
`instance_exec` when given a proc, and most scopes will look something
like `has_many :foos, -> { where(foo: :bar) }`, if we're not capturing
the return value, it's a no-op. However, removing the `merge` causes
`unscope` to break.

While we're merging in the rest of the chain elsewhere, we were never
merging in `unscope` values, causing a breakage on associations where a
default scope was being unscoped in an association scope (yuk!). This is
subtly related to #20722, since it appears we were previously relying on
this mutability.

Fixes #20721.
Fixes #20727.

Closes #20743.

The task `db:_dump` now only dumps the schema if
`ActiveRecord::Base.dump_schema_after_migration` is true. This has
effects:

- `db:migrate:up`
- `db:migrate:down`
- `db:forward`
- `db:rollback`

We recommend using the `bin/` executables in our docs and guides.
Let's make sure that our tests execute the same code path.

Improve integration_tool documentation

As per railties/lib/rails/test_unit/railtie.rb, where it sets it explicitly.

This can be confirmed by starting a new Rails console session in a new app and running this code:

Rails.application.config.app_generators.rails[:integration_tool]

I also beefed up the documentation slightly to be more explicit about what this configuration setting does.

To fix #20644

fix pluralization in doc [ci skip]

Minor addition to getting started guide for foreign_key description

Minor addition to getting started guide for foreign_key description of references migration [ci skip]

Added a note on development kit for Windows Users.  This is required as Windows users can get frustrated when their first rails project creation would fail as some native gem won't get built.

These classes are part of Active Record Preloader, which is not part of
the public API.

fix NoMethodError that occurs when generating scaffold inside full mode engine

Passing nil to image_tag

Fix cache issue when different partials use the same collection

Adds the `virtual_path` option to `cache_fragment_name` so it can
be provided when needed.

That allows `cache_collection_render` to get the appropriate cache
key with the digest generated based on the template and prevent
collision with other templates that cache the same collection.

[ci skip] Fix action_controller_overview

Closes #20705.

With this change, we will always assume the association name is the same
as the table it's referencing. This is subtly different than treating
the hash key passed to `where` as the table name, as it still allows the
class referenced by the association to provide additional type
information.

After exploring several possible solutions to the ambiguity problem, I
do not think there is a short term answer that will maintain backwards
compatibility.

This change will make it so the following code does not work:

    class User
      has_many :approved_posts, -> { where(approved: true) }, class_name: "Post"
    end

    User.where(approved_posts: { id: 1 })

But prevents potential ambiguity and collision as demonstrated in [this
gist](https://gist.github.com/senny/1ae4d8ea7b0e269ed7a0).

Unfortunately, truely solving this requires significantly
re-architecting this code, so that what is currently represented as an
`Arel::Attribute` is instead another data structure that also references
the association it is representing, so we can identify the proper table
name for aliasing when we construct the final tree.

While I'd still like to accomplish that in the long run, I don't think
I'll be able to get there in time for Rails 5 (since I'm not full time
OSS any more, and this is several weeks worth of work). I'm hoping to
achieve this for Rails 5.1.

Fixes #20308