Add specs for Queue. Split it to UniqQueue and ObjectQueue
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```ruby
require "spec_helper"
require "save_queue/object_queue"

describe SaveQueue::ObjectQueue do
  let(:queue)   { SaveQueue::ObjectQueue.new }
  let(:element) { new_element(:element)       }

  [:add, :<<, :push].each do |method|
    describe "##{method}" do
      #it "should add only objects that implement SaveQueue::Object interface" do
      it "should not accept objects that does not respond to #save" do
        element.unstub(:save)
        expect{ queue.add element }.to raise_error ArgumentError, "#{element.inspect} does not respond to #save"
      end

      it "should not accept objects that does not respond to #has_unsaved_changes?" do
        element.unstub(:has_unsaved_changes?)
        expect{ queue.add element }.to raise_error ArgumentError, "#{element.inspect} does not respond to #has_unsaved_changes?"
      end
    end
  end

  describe "#save" do
    it "should save all object in queue" do
      5.times do
        element = stub(:element)
        element.stub(:has_unsaved_changes?).and_return(true)
        element.should_receive(:save).once
        queue << element
      end

      queue.save
    end

    it "should save an object if it has unsaved changes" do
      element = stub(:element)
      element.stub(:has_unsaved_changes?).and_return(true)
      element.should_receive(:save).once

      queue << element
      queue.save
    end

    it "should not save an object if it has not been changed" do
      element = stub(:element)
      element.stub(:has_unsaved_changes?).and_return(false)
      element.should_not_receive(:save)

      queue << element
      queue.save
    end

    it "should raise SaveQueue::FailedSaveError if at least one object in queue was not saved" do
      objects ={}
      objects[:valid1]               = new_element(:valid1)
      objects[:valid2]               = new_element(:valid2)
      objects[:not_changed]          = new_element(:not_changed,         :changed => false, :saved => true)
      objects[:unsaved_but_changed]  = new_element(:unsaved_but_changed, :changed => true,  :saved => false)
      objects[:saved]                = new_element(:saved,               :changed => true,  :saved => true)
      objects[:valid3]               = new_element(:valid3)

      objects.each_value do |object|
        queue << object
      end


      expect{  queue.save }.to raise_error(SaveQueue::FailedSaveError) {|error| \
        error.context.should == { :processed  => objects.values_at(:valid1, :valid2, :not_changed),
                                  :saved      => objects.values_at(:valid1, :valid2),
                                  :failed     => objects[:unsaved_but_changed],
                                  :pending    => objects.values_at(:not_changed, :saved, :valid3) }
      }
    end
  end
end```
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