Add docs with usage examples for ActiveModel::Model

Also add test to ensure basic model does not explode when
initialized with nil.

activemodel/README.rdoc

@@ -13,6 +13,25 @@ in code duplication and fragile applications that broke on upgrades. Active
 Model solves this by defining an explicit API. You can read more about the
 API in ActiveModel::Lint::Tests.
 
+Active Model provides a default module that implements the basic API required
+to integrate with Action Pack out of the box: +ActiveModel::Model+.
+
+    class Person
+      include ActiveModel::Model
+
+      attr_accessor :name, :age
+      validates_presence_of :name
+    end
+
+    person = Person.new(:name => 'bob', :age => '18')
+    person.name # => 'bob'
+    person.age # => 18
+    person.valid? # => false
+
+It includes model name instrospection, conversions, translations and
+validations, resulting in a class suitable to be used with ActionPack.
+See +ActiveModel::Model+ for more examples.
+
 Active Model also provides the following functionality to have ORM-like
 behavior out of the box:
 

activemodel/lib/active_model/model.rb

@@ -1,4 +1,58 @@
 module ActiveModel
+
+  # == Active Model Basic Model
+  #
+  # Includes the required interface for an object to interact with +ActionPack+,
+  # using different +ActiveModel+ modules. It includes model name instrospection,
+  # conversions, translations and validations . Besides that, it allows you to
+  # initialize the object with a hash of attributes, pretty much like
+  # +ActiveRecord+ does.
+  #
+  # A minimal implementation could be:
+  #
+  #   class Person
+  #     include ActiveModel::Model
+  #     attr_accessor :name, :age
+  #   end
+  #
+  #   person = Person.new(:name => 'bob', :age => '18')
+  #   person.name # => 'bob'
+  #   person.age # => 18
+  #
+  # Note that, by default, +ActiveModel::Model+ implements +persisted?+ to
+  # return +false+, which is the most common case. You may want to override it
+  # in your class to simulate a different scenario:
+  #
+  #   class Person
+  #     include ActiveModel::Model
+  #     attr_accessor :id, :name
+  #
+  #     def persisted?
+  #       self.id == 1
+  #     end
+  #   end
+  #
+  #   person = Person.new(:id => 1, :name => 'bob')
+  #   person.persisted? # => true
+  #
+  # Also, if for some reason you need to run code on +initialize+, make sure you
+  # call super if you want the attributes hash initialization to happen.
+  #
+  #   class Person
+  #     include ActiveModel::Model
+  #     attr_accessor :id, :name, :omg
+  #
+  #     def initialize(attributes)
+  #       super
+  #       @omg ||= true
+  #     end
+  #   end
+  #
+  #   person = Person.new(:id => 1, :name => 'bob')
+  #   person.omg # => true
+  #
+  # For more detailed information on other functionality available, please refer
+  # to the specific modules included in +ActiveModel::Model+ (see below).
   module Model
     def self.included(base)
       base.class_eval do
@@ -11,7 +65,7 @@ module ActiveModel
 
     def initialize(params={})
       params.each do |attr, value|
-        self.send(:"#{attr}=", value)
+        self.send("#{attr}=", value)
       end if params
     end
 

activemodel/test/cases/model_test.rb

@@ -16,4 +16,11 @@ class ModelTest < ActiveModel::TestCase
     object = BasicModel.new(:attr => "value")
     assert_equal object.attr, "value"
   end
+
+  def test_initialize_with_nil_or_empty_hash_params_does_not_explode
+    assert_nothing_raised do
+      BasicModel.new()
+      BasicModel.new({})
+    end
+  end
 end