Fix validation callbacks on multiple context

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"]
```

1 files changed, 4 insertions, 0 deletions

diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 2dfde11707..b67a803b9d 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Fix to working before/after validation callbacks on multiple contexts.
+
+    *Yoshiyuki Hirano*
+
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
 *   No changes.