Commit 3dbedd28 added NOT NULL constraints to timestamps.
Commit fcef7289 started to revert this, but was incomplete.
With this commit, 3dbedd28 should be fully reverted and
timestamps will no longer default to NOT NULL.

Integration's definition of #to_param must override
Conversion's. Otherwise, there is a regression from
3.1 in the behavior of a non-persisted AR::Base instance
which nevertheless has an id.

For instance, running

    rails g migration CreateMediaJoinTable artists musics:uniq

will create a migration with

    create_join_table :artists, :musics do |t|
      # t.index [:artist_id, :music_id]
      t.index [:music_id, :artist_id], unique: true
    end

Fixes issue with overrding ActiveRecord reader methods with a
composed object and using that attribute as the scope of a
validates_uniqueness_of validation.

Conflicts:
	activerecord/test/cases/query_cache_test.rb

Dry up reseting the renamed table after each test.

Also made use of the AR::Base.connection object already
available from AR::MigrationTest#connection.

While it's interesting to have the results array, it can make a console or a webpage freeze if there are a lot of them.
So this limits the number of records displayed in #inspect to 10 and tells how much were effectively found.

Fixes #867

The reason for removing the previous implementation of `#inspect` was
that it hid from you that you were dealing with a `Relation` rather than
an `Array`.

But it is still useful to be able to see the records, particularly if you're
writing something like the following in tests:

    assert_equal [foo], Post.where(:bar)

If the assertion fails, you want to see what records were actually
loaded.

So this implementation makes it clear that you've got a `Relation`, but
also shows your records.

This patch fixes quoting for ActiveSupport::Duration instances:

    # before
    >> ActiveRecord::Base.connection.quote 30.minutes
    => "'--- 1800\n...\n'"

    # after
    >> ActiveRecord::Base.connection.quote 30.minutes
    => "1800"

Also, adds a test for type casting ActiveSupport::Duration instances.

Related to #1119.

Mocha is already required by AS::TestCase, so remove the duplicate
requires.

Examples:

    add_reference :products, :supplier, polymorphic: true, index: true
    remove_reference :products, :user

`add_belongs_to` and `remove_belongs_to` are
acceptable.

connection.tables returns an array of strings

and create_join_table_test

Also fix failures in check options for nil

Since composed_of was removed in 05174744,
these tests were working "by mistake", due to the matching "address"
string in the error message, but with a different error message than the
expected multiparameter assignment error.

Since "address" is not an attribute from Customer anymore, the error was
"undefined method klass for nil", where nil was supposed to be the
column object.

All other multiparameter assignment tests are in the same file, so it
makes sense to have all of them here.

does not have TIME data type.