title: mock_s3 decorating pytest fixture
body: <p>I wonder why <code>mock_s3</code> decorator doesn't work when used as a decorator for pytest fixture. <code>test_with_fixture</code> fails while it provides the same code as the <code>test_without</code> fixture. Well, "the same" as it is decorated explicitly.</p>
  
  <p><code>test_with_fixture</code> raises <code>AccessDenied</code> error, but the type of S3 error it not relevant in this case. The problem is that, client.list_objects is not mocked in the test which uses fixture.</p>
  
  <blockquote>
    <p>pytest - 3.1.2<br>
    moto - 1.0.1<br>
    boto3 - 1.0.4</p>
  </blockquote>
  
  <pre><code>import pytest
  import boto3
  
  from moto import mock_s3
  
  BUCKET = 'Foo'
  
  
  @pytest.fixture()
  @mock_s3
  def moto_boto():
      res = boto3.resource('s3')
      res.create_bucket(Bucket=BUCKET)
  
  
  def test_with_fixture(moto_boto):
      client = boto3.client('s3')
      client.list_objects(Bucket=BUCKET)
  
  
  @mock_s3
  def test_without_fixture():     
      res = boto3.resource('s3')
      res.create_bucket(Bucket=BUCKET)
  
      client = boto3.client('s3')
      client.list_objects(Bucket=BUCKET)
  </code></pre>