changed the null check for 'data' parameter to be more explicit.

removed the superfluous encode from the testcase.

requests/models.py

@@ -414,7 +414,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         content_type = None
         length = None
 
-        if not data and json is not None:
+        if data == {} and json is not None:
             content_type = 'application/json'
             body = complexjson.dumps(json)
 

test_requests.py

@@ -1065,7 +1065,7 @@ class RequestsTestCase(unittest.TestCase):
     def test_json_param_post_should_not_override_data_param(self):
         r = requests.Request(method='POST', url='http://httpbin.org/post',
                              data={'stuff'.encode('utf-8'): 'elixr'},
-                             json={'music'.encode('utf-8'): 'flute'})
+                             json={'music': 'flute'})
         prep = r.prepare()
         assert 'stuff=elixr' == prep.body