fix and test for a bug dhagrow reported

requests/models.py

@@ -570,7 +570,9 @@ class Request(object):
 
             # Skip if 'cookie' header is explicitly set.
             if 'cookie' not in self.headers:
-                self.headers['Cookie'] = get_cookie_header(self.cookies, self)
+                cookie_header = get_cookie_header(self.cookies, self)
+                if cookie_header is not None:
+                    self.headers['Cookie'] = cookie_header
 
             # Pre-request hook.
             r = dispatch_hook('pre_request', self.hooks, self)

tests/test_cookies.py

@@ -81,6 +81,11 @@ class CookieTests(TestBaseMixin, unittest.TestCase):
         self.assertEqual(json.loads(r2.text)['cookies'], expected_cookies)
         self.assertIs(r.cookies, r2.cookies)
 
+    def test_none_cookie(self):
+        """Regression test: don't send a Cookie header with a string value of 'None'!"""
+        page = json.loads(requests.get(httpbin('headers')).text)
+        self.assertNotIn('Cookie', page['headers'])
+
 class LWPCookieJarTest(TestBaseMixin, unittest.TestCase):
     """Check store/load of cookies to FileCookieJar's, specifically LWPCookieJar's."""