Change Virtus::InstanceMethods#initialize to not create an empty hash…

… by default

* When no arguments are passed in, the end result is the same as when a nil is
  passed in, so there's no need to create an empty Hash and then "set" the
  internal state with it. Better to just default to nil, and short-circuit the
  call.
* Added specs so that if anything other than nil or an object that responds to
  #to_hash is passed in an exception is raised. Like DM1, by default I *only*
  want constructors handling Hash-like objects or nil; anything else passed in
  should be considered an exceptional case.

lib/virtus/instance_methods.rb

@@ -11,8 +11,8 @@ module InstanceMethods
     # @return [undefined]
     #
     # @api private
-    def initialize(attributes = {})
-      self.attributes = attributes if attributes.respond_to?(:to_hash)
+    def initialize(attributes = nil)
+      self.attributes = attributes if attributes
     end
 
     # Returns a value of the attribute with the given name

spec/unit/virtus/instance_methods/initialize_spec.rb

@@ -8,25 +8,35 @@
     end
   end
 
-  let(:subject) do
-    described_class.new(attributes)
-  end
+  context 'with Hash argument' do
+    subject { described_class.new(:name => name) }
 
-  context "with valid argument" do
-    let(:attributes) do
-      {:name => 'john'}
-    end
+    let(:name) { stub('name') }
 
     it 'sets attributes' do
-      subject.name.should == attributes[:name]
+      subject.name.should equal(name)
     end
   end
 
-  context "with 'non-hashy' argument" do
-    let(:attributes) { '' }
+  context 'with nil argument' do
+    subject { described_class.new(nil) }
 
-    it 'does not try to set attributes' do
-      expect { subject }.to_not raise_error(NoMethodError)
+    it 'does not set attributes' do
+      subject.name.should be_nil
     end
   end
+
+  context 'with no arguments' do
+    subject { described_class.new }
+
+    it 'does not set attributes' do
+      subject.name.should be_nil
+    end
+  end
+
+  context' with argument that does not respond to #to_hash' do
+    subject { described_class.new(Object.new) }
+
+    specify { expect { subject }.to raise_error(NoMethodError) }
+  end
 end