Fix serialization for overridden fields.

- When a declared field has it's getter overridden, we return the value
  of the overridden method and not of read_attribute.

- Closes #2023.

lib/mongoid/serialization.rb

@@ -49,8 +49,8 @@ def serializable_hash(options = nil)
         (attribute_names + method_names).each do |name|
           if relations.has_key?(name)
             value = send(name)
-            attrs[name] = value.serializable_hash(options)
-          elsif attribute_names.include?(name.to_s)
+            attrs[name] = value ? value.serializable_hash(options) : nil
+          elsif attribute_names.include?(name) && !fields.has_key?(name)
             attrs[name] = read_attribute(name)
           else
             attrs[name] = send(name)

spec/app/models/person.rb

@@ -27,6 +27,7 @@ class Person
   field :security_code
   field :reading, :type => Object
   field :bson_id, :type => BSON::ObjectId
+  field :override_me, :type => Integer
 
   index :age
   index :addresses
@@ -147,6 +148,10 @@ def savable?
     self.mode != :prevent_save
   end
 
+  def override_me
+    read_attribute(:override_me).to_s
+  end
+
   class << self
     def accepted
       criteria.where(:terms => true)

spec/functional/mongoid/serialization_spec.rb

@@ -60,6 +60,21 @@
       end
     end
 
+    context "when the method for a declared field is overridden" do
+
+      before do
+        person.override_me = 1
+      end
+
+      let(:attributes) do
+        person.serializable_hash
+      end
+
+      it "uses the overridden method" do
+        attributes["override_me"].should eq("1")
+      end
+    end
+
     context "when the model has embedded documents" do
 
       let!(:address) do