Add specs for references commands.
user
```ruby
# encoding: utf-8

require 'spec_helper'

describe GithubCLI::Commands::References do
  let(:format) { 'table' }
  let(:user)   { 'peter-murach' }
  let(:repo)   { 'github_cli' }
  let(:ref)    { 'refs/master' }
  let(:sha)    { '3a0f86fb8db8eea7ccbb9a95f325ddbedfb25e15' }
  let(:api_class) { GithubCLI::Reference }

  it "invokes ref:list" do
    api_class.should_receive(:list).with(user, repo, {}, format)
    subject.invoke "ref:list", [user, repo]
  end

  it "invokes ref:list --ref" do
    api_class.should_receive(:list).with(user, repo, {"ref" => ref}, format)
    subject.invoke "ref:list", [user, repo], :ref => ref
  end

  it "invokes ref:get" do
    api_class.should_receive(:get).with(user, repo, ref, {}, format)
    subject.invoke "ref:get", [user, repo, ref]
  end

  it "invokes ref:create --ref --sha" do
    api_class.should_receive(:create).with(user, repo, {"ref" => ref, "sha" => sha}, format)
    subject.invoke "ref:create", [user, repo], :ref => ref, :sha => sha
  end

  it "invokes ref:update --sha -f" do
    api_class.should_receive(:update).with(user, repo, ref, {"sha" => sha, "force" => false}, format)
    subject.invoke "ref:update", [user, repo, ref], :sha => sha
  end

  it "invokes ref:delete" do
    api_class.should_receive(:delete).with(user, repo, ref, {}, format)
    subject.invoke "ref:delete", [user, repo, ref]
  end
end
```
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