Added a Prependable specs

spec/lib/prependable_spec.rb

+require 'spec_helper'
+
+describe Prependable do
+  subject { FooObject }
+
+  context 'class methods' do
+    it "has a method" do
+      expect(subject).to respond_to(:class_value)
+    end
+
+    it 'can execute a method' do
+      expect(subject.class_value).to eq(200)
+    end
+  end
+
+  context 'instance methods' do
+    it "has a method" do
+      expect(subject.new).to respond_to(:value)
+    end
+
+    it 'chains a method execution' do
+      expect(subject.new.value).to eq(100)
+    end
+  end
+
+  module Foo
+    extend Prependable
+
+    prepended do
+      def self.class_value
+        20
+      end
+    end
+
+    def value
+      super * 10
+    end
+  end
+
+  class FooObject
+    prepend Foo
+
+    def value
+      10
+    end
+  end
+end