Fix spec

Author: johnnyshields

spec/mongoid/fields_spec.rb

@@ -1787,9 +1787,12 @@ class DiscriminatorChild2 < DiscriminatorParent
   describe '.configure DSL' do
 
     context '.type method' do
-      after do
+      around do |example|
         klass = Mongoid::Fields::FieldTypes
-        klass.instance_variable_set(:@mapping, klass::DEFAULT_MAPPING.dup)
+        mapping = klass::DEFAULT_MAPPING.dup
+        klass.instance_variable_set(:@mapping, mapping)
+        example.run
+        klass.instance_variable_set(:@mapping, mapping)
       end
 
       it 'can define a custom type' do