Fix ActiveModel::Errors#delete return value to stay backward compatible

Rails 5.0 changes to ActiveModel::Errors include addition of `details`
that also accidentally changed the return value of `delete`. Since
there was no test for that behavior it went unnoticed. This commit
adds a test and fixes the regression.

Small improvements to comments have also been made. Since `get` is
getting deprecated it is better to use `[]` in other methods' code
examples. Also, in the module usage example, `def Person.method`
was replaced with a more commonly used `def self.method` code style.

1 files changed, 6 insertions, 0 deletions

diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index da142ea2c0..27631005e5 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -377,6 +377,12 @@ class ErrorsTest < ActiveModel::TestCase
     assert_empty errors.details[:name]
   end
 
+  test "delete returns the deleted messages" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :invalid)
+    assert_equal errors.delete(:name), ["is invalid"]
+  end
+
   test "clear removes details" do
     person = Person.new
     person.errors.add(:name, :invalid)