stats on by default make this test fail: 

```
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 332, in run
    testMethod()
  File "/home/paul/dev/scrapy-dev/scrapy/tests/test_spidermiddleware_offsite.py", line 33, in test_process_spider_output
    out = list(self.mw.process_spider_output(res, reqs, self.spider))
  File "/home/paul/dev/scrapy-dev/scrapy/contrib/spidermiddleware/offsite.py", line 38, in process_spider_output
    spider.crawler.stats.inc_value('offsite/domains',
  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

scrapy.tests.test_spidermiddleware_offsite.TestOffsiteMiddleware.test_process_spider_output
-------------------------------------------------------------------------------

```