useragent: if spider user_agent attribute is None, use default useragent instead of not including one

--HG--
extra : convert_revision : svn%3Ab85faa78-f9eb-468e-a121-7cced6da292c%401095

scrapy/trunk/scrapy/contrib/downloadermiddleware/useragent.py

@@ -9,6 +9,6 @@ class UserAgentMiddleware(object):
     default_useragent = settings.get('USER_AGENT')
 
     def process_request(self, request, spider):
-        ua = getattr(spider, 'user_agent', self.default_useragent)
+        ua = getattr(spider, 'user_agent', None) or self.default_useragent
         if ua:
             request.headers.setdefault('User-Agent', ua)

scrapy/trunk/scrapy/tests/test_downloadermiddleware_useragent.py

@@ -23,7 +23,14 @@ class CookiesMiddlewareTest(TestCase):
         assert self.mw.process_request(req, self.spider) is None
         self.assertEquals(req.headers['User-Agent'], 'default_useragent')
 
+        # None or not present user_agent attribute is the same
+        self.spider.user_agent = None
+        req = Request('http://scrapytest.org/')
+        assert self.mw.process_request(req, self.spider) is None
+        self.assertEquals(req.headers['User-Agent'], 'default_useragent')
+
     def test_spider_agent(self):
+        self.mw.default_useragent = 'default_useragent'
         self.spider.user_agent = 'spider_useragent'
         req = Request('http://scrapytest.org/')
         assert self.mw.process_request(req, self.spider) is None