Fix destructive side effects from marshaling an association caused by 65843e1a

Related to 761bc751 and
eb876c4d

This reverts commit 761bc751.

This commit wasn't fixing any issue just using the same table for
different models with different primary keys.

Use default charset/collation only if *neither* is specified. Otherwise we set utf8_unicode_ci collation on latin1 table.

If your database supports setting the isolation level for a transaction,
you can set it like so:

  Post.transaction(isolation: :serializable) do
    # ...
  end

Valid isolation levels are:

* `:read_uncommitted`
* `:read_committed`
* `:repeatable_read`
* `:serializable`

You should consult the documentation for your database to understand the
semantics of these different levels:

* http://www.postgresql.org/docs/9.1/static/transaction-iso.html
* https://dev.mysql.com/doc/refman/5.0/en/set-transaction.html

An `ActiveRecord::TransactionIsolationError` will be raised if:

* The adapter does not support setting the isolation level
* You are joining an existing open transaction
* You are creating a nested (savepoint) transaction

The mysql, mysql2 and postgresql adapters support setting the
transaction isolation level. However, support is disabled for mysql
versions below 5, because they are affected by a bug
(http://bugs.mysql.com/bug.php?id=39170) which means the isolation level
gets persisted outside the transaction.

Callbacks may interfere with each other.

Breaks db:setup because it tries to load the environment before creating the database.

This reverts commit 5ca11fef.

Closes #6960

Remove unused private method AR::NestedAttributes#unassignable_keys and reference to mass_assignment options

Update AR::Base.create and AR:Base#update_attributes docs to remove references to mass assignment options

If assigning to a has_many :through collection against an unsaved
object using the collection=[<array_of_items>] syntax, the join models
were not properly created, previously.

attr_accessible and attr_protected raise an exception pointing to use plugin or new protection model

The COUNT clause of a finder_sql relationship is being rewritten from
COUNT(*) to COUNT(table_name.*). This does not appear to be valid syntax
in MySQL:

```
mysql> SELECT COUNT( table_name.* ) FROM `table_name`;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near '* ) FROM `table_name`' at line 1
```

This fixes the bug, as well as adding tests so we don't re-introduce
it in the future.

Fixes #3956.

I found them when I was running
warning mode on with railties

See https://github.com/rails/rails/pull/3782

Fixes #3458.

Accidentally checked in commented test code. Fail. >_<