Running the tests without it raises an `AssertionError`

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 323, in run
    self.setUp()
  File "/home/paul/dev/scrapy-dev/scrapy/tests/test_spidermiddleware_offsite.py", line 13, in setUp
    self.mw.spider_opened(self.spider)
  File "/home/paul/dev/scrapy-dev/scrapy/contrib/spidermiddleware/offsite.py", line 67, in spider_opened
    spider.crawler.stats
  File "/home/paul/dev/scrapy-dev/scrapy/spider.py", line 41, in crawler
    assert hasattr(self, '_crawler'), "Spider not bounded to any crawler"
exceptions.AssertionError: Spider not bounded to any crawler
```