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, 43 insertions, 0 deletions

diff --git a/activemodel/test/cases/validations/callbacks_test.rb b/activemodel/test/cases/validations/callbacks_test.rb
index d3a9a17a05..ff3cf61746 100644
--- a/activemodel/test/cases/validations/callbacks_test.rb
+++ b/activemodel/test/cases/validations/callbacks_test.rb
@@ -59,6 +59,18 @@ class DogValidatorWithOnCondition < Dog
   def set_after_validation_marker;  history << "after_validation_marker" ; end
 end
 
+class DogValidatorWithOnMultipleCondition < Dog
+  before_validation :set_before_validation_marker_on_context_a, on: :context_a
+  before_validation :set_before_validation_marker_on_context_b, on: :context_b
+  after_validation :set_after_validation_marker_on_context_a, on: :context_a
+  after_validation :set_after_validation_marker_on_context_b, on: :context_b
+
+  def set_before_validation_marker_on_context_a; history << "before_validation_marker on context_a"; end
+  def set_before_validation_marker_on_context_b; history << "before_validation_marker on context_b"; end
+  def set_after_validation_marker_on_context_a;  history << "after_validation_marker on context_a" ; end
+  def set_after_validation_marker_on_context_b;  history << "after_validation_marker on context_b" ; end
+end
+
 class DogValidatorWithIfCondition < Dog
   before_validation :set_before_validation_marker1, if: -> { true }
   before_validation :set_before_validation_marker2, if: -> { false }
@@ -98,6 +110,37 @@ class CallbacksWithMethodNamesShouldBeCalled < ActiveModel::TestCase
     assert_equal [], d.history
   end
 
+  def test_on_multiple_condition_is_respected_for_validation_with_matching_context
+    d = DogValidatorWithOnMultipleCondition.new
+    d.valid?(:context_a)
+    assert_equal ["before_validation_marker on context_a", "after_validation_marker on context_a"], d.history
+
+    d = DogValidatorWithOnMultipleCondition.new
+    d.valid?(:context_b)
+    assert_equal ["before_validation_marker on context_b", "after_validation_marker on context_b"], d.history
+
+    d = DogValidatorWithOnMultipleCondition.new
+    d.valid?([:context_a, :context_b])
+    assert_equal([
+      "before_validation_marker on context_a",
+      "before_validation_marker on context_b",
+      "after_validation_marker on context_a",
+      "after_validation_marker on context_b"
+    ], d.history)
+  end
+
+  def test_on_multiple_condition_is_respected_for_validation_without_matching_context
+    d = DogValidatorWithOnMultipleCondition.new
+    d.valid?(:save)
+    assert_equal [], d.history
+  end
+
+  def test_on_multiple_condition_is_respected_for_validation_without_context
+    d = DogValidatorWithOnMultipleCondition.new
+    d.valid?
+    assert_equal [], d.history
+  end
+
   def test_before_validation_and_after_validation_callbacks_should_be_called
     d = DogWithMethodCallbacks.new
     d.valid?