Fix a bug affecting validations of enum attributes

This fixes a bug where any enum attribute of a model
would be evaluated always as 0 when calling the
database on validations.

This fix converts the value of the enum attribute
to its integer value rather than the string before
building the relation as the bug occured when the
string finally gets converted to integer using
string.to_i which converts it to 0.

[Vilius Luneckas, Ahmed AbouElhamayed]

fix test cases

Active Record is specifically for databases. Refs #12101.

Cache#fetch_multi returns a Hash instead of an Array

The current implementation of `fetch_multi` returns an array and has no
means to easily backtrack which names yielded which results. By changing
the return value to a Hash we retain the name information. Hash#values
can be used on the response if only the values are needed.

[skip ci] Fixes docs typo in nested resource path helpers

Now that Validator #setup is called from the initializer, we need a
reference to the model's class to be passed in to allow the validators
to continue functioning when used at the instance level.

Closes #14134.

Goes along with fea1cdcf and
59ec4562.

Fix controller test not resetting @_url_options

Benchmark:

Benchmark.ips do |x|
  x.report("home")       { "home".underscore }
  x.report("Home")       { "Home".underscore }
  x.report("homeBase")   { "homeBase".underscore }
  x.report("home::base") { "home::base".underscore }
end

Before:

Calculating -------------------------------------
                home      2598 i/100ms
                Home      2463 i/100ms
            homeBase      2300 i/100ms
          home::base      2086 i/100ms
-------------------------------------------------
                home    28522.3 (±14.7%) i/s -     140292 in   5.065102s
                Home    29165.8 (±14.9%) i/s -     140391 in   5.000475s
            homeBase    26218.5 (±7.1%) i/s -     131100 in   5.030485s
          home::base    27712.3 (±5.9%) i/s -     139762 in   5.064077s

After:

Calculating -------------------------------------
                home     23163 i/100ms
                Home      2432 i/100ms
            homeBase      2160 i/100ms
          home::base      2265 i/100ms
-------------------------------------------------
                home  1501614.8 (±10.2%) i/s -    7412160 in   5.009540s
                Home    28754.0 (±8.5%) i/s -     143488 in   5.033886s
            homeBase    25331.1 (±5.6%) i/s -     127440 in   5.047940s
          home::base    27089.9 (±5.5%) i/s -     135900 in   5.033516s

We should not rely on to_s to return the name of the class

[ci skip] Update list of files extensions in rake notes guide

Commit 4f2cd3e9 introduced a bug by reordering the call to
`@controller.recycle!` above the call to `build_request_uri`. The
impact of this was that the `@_url_options` cache ends up not being
reset between building a request URI (occurring within the test
controller) and the firing of the actual request.

We encountered this bug because we had the following setup:

  class MinimumReproducibleController < ActionController::Base
    before_filter { @param = 'param' }

    def index
      render text: url_for(params)
    end

    def default_url_options
      { custom_opt: @param }
    end
  end

  def test_index
    get :index # builds url, then fires actual request
  end

The first step in  `get :index` in the test suite would populate the
@_url_options cache. The subsequent call to `url_for` inside of the
controller action would then utilize the uncleared cache, thus never
calling the now-updated default_url_options.

This commit fixes this bug calling recycle! twice, and removes a call
to set response_body, which should no longer be needed since we're
recycling the request object explicitly.

[ci skip] Move association class method notes

Make explanation of association class methods clearer by moving notes to beginning of each example section.

[ci skip] remove guide section on models in migrations [ci skip]

Both examples given are incorrect, and no one can seem to come up with
a realistic situation where this is an actual problem. For now, it seems like
the best thing to do is just scrap the section altogether.

This test currently outputs the following warning:

```
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"   ORDER BY "postgresql_enums"."id" ASC LIMIT 1)
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"  WHERE "postgresql_enums"."id" = $1 LIMIT 1)
```

We have an open PR to deal with this issue. It will dynamically
register the OID for enum columns. This test case is merely to exhibit
the current behavior of PostgreSQL enum columns.

Bump version of bcrypt gem

From #14026:

    Specific rbx-2 to limit testing on Rubinius 2.x (since there will be
    other versions of Rubinius > 2.x soon).

    Also, as of Rubinius 2.2.5, it is no longer necessary to bundle the
    rubysl gem.

This is what Rails master/4.1 supports, so we don't need to add rubysl to
gemfiles anymore.

`before_add` callbacks are fired before the record is saved on
`has_and_belongs_to_many` assocations *and* on `has_many :through`
associations.  Before this change, `before_add` callbacks would be fired
before the record was saved on `has_and_belongs_to_many` associations, but
*not* on `has_many :through` associations.

Fixes #14144

Fix test name typo in app generator tests.

Conflicts:
	activerecord/CHANGELOG.md

travis s/2.1.0/2.1.1

Handle missing environment from non empty config

Skip test_migrate_revert_add_index_with_name for some databases

do not allow to create duplicate indexes on the same columns

We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/