E.G.:

```ruby
class Parent < ActiveRecord::Base
  has_one :child
  validates_presence_of :name, :on => "custom_context"
  validates_associated :child
end

class Child < ActiveRecord::Base
  belongs_to :parent
  validates_presence_of :name, :on => "custom_context"
end

p = Parent.new(:name => "Montoto", :child => Child.new)
p.valid?(:custom_context) # => Returns true, even though the child is not valid under the same context.
```

If a model belongs_to two associations with the same class, then reset_counters
will reset the wrong counter cache.

Finding the right reflection should use the foreign_key instead, which should
be unique.

See the CHANGELOG for details.

Fixes #950.

This reverts commit c99d507f.

TL;DR: MySQL sucks.

Since it does not support transactions for schema changes,
when you run `create_table` in setup block, it will commit
current transaction. Now tests want to run `RELEASE SAVEPOINT`
after finishing tests which fails since there is no transaction.