Add specs for class Tag.
user
```ruby
require "rspec"
require "gmail/tag"

class DataError < Exception; end

describe Tag do

  before(:all) do
    @tags = [ "Perso", "Perso/Foo" ]
    @tag1 = Tag.new(@tags[0])
    @tag2 = Tag.new(@tags[1])
  end

  describe "#initialize" do

    it "should raise an exception if @tag is nil" do
      expect{@tag = Tag.new}.to raise_error(ArgumentError)
    end

    it "should create a @tag object" do
      @tag1.should_not be_nil
      @tag2.should_not be_nil
      @tag1.should be_an_instance_of(Tag)
      @tag2.should be_an_instance_of(Tag)
    end

    it "should have a label member" do
      @tag1.label.should be_instance_of(String)
      @tag2.label.should be_instance_of(String)
    end

    it "should have the right label value" do
      @tag1.label.should == "Perso"
      @tag2.label.should == "Perso/Foo"
    end
  end

  describe "#normalize" do
    it "should keep the right value if no '/'" do
      @tag1.normalize.should == "Perso"
    end

    it "should convert '/' into '-'" do
      @tag2.normalize.should == "Perso-Foo"
    end
  end
end
```
assistant