I found a bug that validation callbacks don't fire on multiple context.
So I've fixed it.
Example:
```ruby
class Dog
include ActiveModel::Validations
include ActiveModel::Validations::Callbacks
attr_accessor :history
def initialize
@history = []
end
before_validation :set_before_validation_on_a, on: :a
before_validation :set_before_validation_on_b, on: :b
after_validation :set_after_validation_on_a, on: :a
after_validation :set_after_validation_on_b, on: :b
def set_before_validation_on_a; history << "before_validation on a"; end
def set_before_validation_on_b; history << "before_validation on b"; end
def set_after_validation_on_a; history << "after_validation on a" ; end
def set_after_validation_on_b; history << "after_validation on b" ; end
end
```
Before:
```
d = Dog.new
d.valid?([:a, :b])
d.history # []
```
After:
```
d = Dog.new
d.valid?([:a, :b])
d.history # ["before_validation on a", "before_validation on b", "after_validation on a", "after_validation on b"]
```
470d0e459f