Allow routing concerns to accept a callable

Users need to know the ultimate problem here is that AS was
trying to autoload a constant and it failed.

custom inheritance_column test refactoring

All tests with a custom inheritance_column use the `Vegtable` model.
The field ruby_type on the Company models is no longer needed

previously the tests with and without a custom `inheritance_column`
used the same models. Since the model then has both fields this can lead
to false positives.

Also, add documentation for alternate usage.

This allows us to make alterations to the generated routes based on the
scope of the current mapper, and otherwise allows us to move larger
blocks of concerns out of the routes file, altogether.

#becomes works with configured inheritance_column

I had to create a new table because I needed an STI table,
which does not have both a "type" and a "custom_type"

the test fails with:
  1) Error:
test_alt_becomes_works_with_sti(InheritanceTest):
NoMethodError: undefined method `type=' for #<Cabbage id: 1, name: "my cucumber", custom_type: "Cucumber">
    /Users/username/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:432:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:100:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/persistence.rb:165:in `becomes'
    test/cases/inheritance_test.rb:134:in `test_becomes_works_with_sti'
    test/cases/inheritance_test.rb:140:in `test_alt_becomes_works_with_sti'

Fixed markup in Upgrading RoR guide [ci skip]

Fix example code: use tasks instead of comments [ci skip]

There are two changes:
- Using <ruby> syntax for unicode character routes example instead of
  <tt> since it causes single quotes to be transformed into inverted
  commas. This makes this required change more eye-catching.
- Using <tt> for the j alias and related helpers as written in the
  documentation guidelines.

Properly reset the session on reset_session

Use load_for_write! to ensure a refresh of the session object.
This way the new session_id and the empty data will be stored properly.
E.g. in the case of the session cookie store this means that a new
digest will be returned to the user.

The app generator is not generally run under bundler, but the Bundler
constant is used here.

In particular you cannot create --dev apps without this.

This reverts commit fc80ebb1.

Such changes aren't allowed in docrails.

ActionView decoupling and sprockets-rails tests fail

Method invalid_asset_host! was delegated to controller but sprockets
compile assets in their own scope without controller. And if we set asset_host
with second parameter it should raise error through invalid_asset_host!.
But since controller is nil it cannot be reached.

Fixes #7478

Option to run `bundle install` after generating new appplication was
added recently to rails. Since introduction, it contained a subtle bug
that caused it to use `Gemfile` from current directory (if it exists)
rather than from generated directory. This also accidentaly caused
railties tests to work without any problems - after generating test app
it just used `Gemfile` from the repository, rather than the one in
generated app. After fixing the bug mentioned above, this of course
broke. The easiest way to bypass that is to not generate a `Gemfile` for
test application - with such setup Bundler will just use first available
`Gemfile` in one of the parent directories.

The cause of the previous revert was bug in bundler that made it hard to
make railties test work. Fix for bundler was recently pushed to github,
so now we can safely get back to the original commit.

This reverts commit 0f5cc34a.

SessionHash isn't a thing, and tenses are wrong.

This is purely a performance optimisation.

See https://gist.github.com/3552829

This is a performance/GC optimisation.

In theory, this could be optimised by the implementation (last time I
checked, this would have no effect on JRuby). But in practise, this make
attribute access faster.

We don't need separate @class_to_pool and @connection_pool hashes.

* Loop rather than recurse in retrieve_connection_pool
* Key the hash by class rather than class name. This avoids creating
  unnecessary strings.

This reverts commit 7256cb53, reversing
changes made to 6ebe22c3.

Reason: A test was failing.