Fix FrozenError on ActiveModel::Error clear and delete

This PR fixes a `FrozenError` when attempting to clear or delete
`ActiveModel::Errors` through the deprecated array methods. In
particular the error happens in the following situations:

    # calling `clear` on the deprecated array
    errors[:some_attribute].clear

    # calling `delete` on the deprecated messages hash
    errors.messages.delete(:some_attribute)

Following the recent introduction of `ActiveModel::Error`, this PR adds
deprecation warnings for those two messages.

activemodel/lib/active_model/errors.rb

@@ -569,6 +569,12 @@ module ActiveModel
       __setobj__ prepare_content
     end
 
+    def delete(attribute)
+      ActiveSupport::Deprecation.warn("Calling `delete` to an ActiveModel::Errors messages hash is deprecated. Please call `ActiveModel::Errors#delete` instead.")
+
+      @errors.delete(attribute)
+    end
+
     private
       def prepare_content
         content = @errors.to_hash
@@ -599,6 +605,12 @@ module ActiveModel
       __setobj__ @errors.messages_for(@attribute)
       self
     end
+
+    def clear
+      ActiveSupport::Deprecation.warn("Calling `clear` to an ActiveModel::Errors message array in order to delete all errors is deprecated. Please call `ActiveModel::Errors#delete` instead.")
+
+      @errors.delete(@attribute)
+    end
   end
 
   class DeprecationHandlingDetailsHash < SimpleDelegator

activemodel/test/cases/errors_test.rb

@@ -103,6 +103,15 @@ class ErrorsTest < ActiveModel::TestCase
     assert_empty person.errors
   end
 
+  test "clear errors by key" do
+    person = Person.new
+    person.validate!
+
+    assert_equal 1, person.errors.count
+    assert_deprecated { person.errors[:name].clear }
+    assert_empty person.errors
+  end
+
   test "error access is indifferent" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, "omg")
@@ -619,6 +628,15 @@ class ErrorsTest < ActiveModel::TestCase
     )
   end
 
+  test "messages delete (deprecated)" do
+    person = Person.new
+    person.validate!
+
+    assert_equal 1, person.errors.count
+    assert_deprecated { person.errors.messages.delete(:name) }
+    assert_empty person.errors
+  end
+
   test "group_by_attribute" do
     person = Person.new
     error = person.errors.add(:name, :invalid, message: "is bad")