closes #8663.

When preloading a hmt association there two possible scenarios:

1.) preload with 2 queries: first hm association, then hmt with id IN ()
2.) preload with join: hmt association is loaded with a join on the hm association

The bug was happening in scenario 1.) with a normal order clause on the hmt association.
The ordering was also applied when loading the hm association, which resulted in the error.

This patch only applies the ordering the the hm-relation if we are performing a join (2).
Otherwise the order will only appear in the second query (1).

remove config.auto_explain_threshold_in_seconds

We discussed that the auto explain feature is rarely used.
This PR removes only the automatic explain. You can still display
the explain output for any given relation using `ActiveRecord::Relation#explain`.

As a side-effect this should also fix the connection problem during
asset compilation (#9385). The auto explain initializer in the `ActiveRecord::Railtie`
forced a connection.

Updated doc to clarify about update_all and delete_all

[ci skip]

Point to the Protected Attributes gem to those who want a smoothly
upgrade path before to change to use Strong Parameters.

[ci skip]

delete_all will call callbacks if depdent option is specified as
:destroy. This behavior is not very clear from the documetation and
hence issue #7832 was raised.

closes #7832

The connection url parssing don't accept the socket option

We should only type cast when we need to use.

Related to 4b005fb3

Revert "move config.assets.precompile into config/application.rb"

This reverts commit 304f4d4c.

As it turns out everything should behave as it did and we don't
need to move this configuration option.

multiple actions for :on option with `after_commit` and `after_rollback`

test case to prevent duplicated associations with custom PK.

closes #9201

Address failure test_rename_column_with_multi_column_index with Oracle

because of Oracle database index length spec.

also rename the test-case class inside columns_test.rb

Reason: on a second thought, minitest prints a seed as run option
regardless of the test order, and it actually calls srand, so
albeit it might be misleading I believe, it is the way it is and
should be left that way.

This reverts commit c15862ae.

See the first FIXME comment in the patch for the rationale.

guides/rails_guides moved up and out of the railties directory 

Forwardport #9388 rename/remove column preserves custom PK.

I renamed the test to better communicate it's intention. Since it also tests:
  - add_column
  - remove_column
  - change_column
There is no reason to call it rename_column_test.

Fix debug helper not inspecting when it can't convert to YAML

The debug helper should inspect the object when it can't be converted to YAML, this behavior was changed in 8f8d8eb1.

Since all changes from #9347 are related to AS, it seems proper that the
test is placed there as well.

Address edge case for number_to_human with units option.

Correct comment for (beginning|end)_of_minute [ci skip]

When we removed script/rails and introduced bin/rails, we accidentally
introduced a regression. If you install Rails 4 as a gem, then try to do
something in a Rails 3 application:

    $ rails g

This will throw the 'please type rails new foo' message rather than the
proper generator documentation message. This is because older apps don't
have bin/rails.

Therefore, we now *prefer* bin/rails, but still search for script/rails,
and exec the one we find.

ActiveSupport::NumberHelper#number_to_human now returns the number unaltered when
the units hash does not contain the needed key, e.g. when the number provided is less
than the largest key provided.

use ISO 8601 format in time_tag helper