Add a simple spec for the azure mixin
user
```ruby
#
# Author:: Tim Smith <tsmith@chef.io>
# Copyright:: 2017 Chef Software, Inc.
# License:: Apache License, Version 2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDIT"Net::HTTP Response"NS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

require_relative "../../spec_helper.rb"
require "ohai/mixin/azure_metadata"

describe Ohai::Mixin::AzureMetadata do
  let(:mixin) do
    mixin = Object.new.extend(::Ohai::Mixin::AzureMetadata)
    mixin
  end

  describe "#http_get" do
    it "gets the passed URI" do
      http_mock = double("http")
      allow(http_mock).to receive(:read_timeout=)
      allow(Net::HTTP).to receive(:start).with("169.254.169.254").and_return(http_mock)

      expect(http_mock).to receive(:get).with('http://www.chef.io', initheader = { "Metadata" => "true" })
      mixin.http_get('http://www.chef.io')
    end
  end

  describe "#fetch_metadata" do
    it "returns an empty hash given a non-200 response" do
      http_mock = double("http", {:code => "500" })
      allow(mixin).to receive(:http_get).and_return(http_mock)

      expect(Ohai::Log).to receive(:warn)
      vals = mixin.fetch_metadata
      expect(vals).to eq({})
    end

    it "returns an empty hash given invalid JSON response" do
      http_mock = double("http", {:code => "200", :body => '{ "foo" "bar"}' })
      allow(mixin).to receive(:http_get).and_return(http_mock)

      expect(Ohai::Log).to receive(:warn)
      vals = mixin.fetch_metadata
      expect(vals).to eq({})
    end

    it "returns a populated hash given valid JSON response" do
      http_mock = double("http", {:code => "200", :body => '{ "foo": "bar"}' })
      allow(mixin).to receive(:http_get).and_return(http_mock)

      expect(Ohai::Log).not_to receive(:warn)
      vals = mixin.fetch_metadata
      expect(vals).to eq({"foo" => "bar"})
    end
  end
end
```
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