Only use type in inheritance situations.

This is mostly a performance related change, but instead of always
having _type field defined and suffering the inheritance check every
time we apply defaults, we now only define it once something has been
subclassed.

The perf implications on 150k iterations:

Before:

Load from DB  4.580000   0.010000   4.590000 (  4.660224)
Instantiation 4.900000   0.000000   4.900000 (  4.904462)

After:

Load from DB  2.840000   0.010000   2.850000 (  2.910107)
Instantiation 3.540000   0.000000   3.540000 (  3.542787)

lib/mongoid/fields.rb

@@ -21,7 +21,6 @@ module Fields
       self.pre_processed_defaults = []
       self.post_processed_defaults = []
 
-      field(:_type, default: ->{ self.class.name if hereditary? }, type: String)
       field(
         :_id,
         default: ->{ Moped::BSON::ObjectId.new },

lib/mongoid/hierarchy.rb

@@ -145,6 +145,13 @@ def inherited(subclass)
         subclass.pre_processed_defaults = pre_processed_defaults.dup
         subclass.post_processed_defaults = post_processed_defaults.dup
         subclass.scopes = scopes.dup
+
+        # We only need the _type field if inheritance is in play, but need to
+        # add to the root class as well for backwards compatibility.
+        unless fields.has_key?("_type")
+          field(:_type, default: self.name, type: String)
+        end
+        subclass.field(:_type, default: subclass.name, type: String)
       end
     end
   end

spec/mongoid/fields_spec.rb

@@ -634,7 +634,7 @@
       end
 
       it "does not return subclass defaults" do
-        shape.pre_processed_defaults.should eq([ "_id", "x", "y" ])
+        shape.pre_processed_defaults.should eq([ "_id", "x", "y", "_type" ])
       end
     end
 
@@ -645,7 +645,7 @@
       end
 
       it "has the parent and child defaults" do
-        circle.pre_processed_defaults.should eq([ "_id", "x", "y", "radius" ])
+        circle.pre_processed_defaults.should eq([ "_id", "x", "y", "_type", "radius" ])
       end
     end
   end