diff --git a/requests/models.py b/requests/models.py
index 49007372..970acf8a 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -34,6 +34,8 @@ from .compat import (
     cookielib, urlparse, urlunparse, urljoin, urlsplit, urlencode, str, bytes,
     StringIO, is_py2, chardet, json)
 
+from __builtin__ import str as builtin_str
+
 REDIRECT_STATI = (codes.moved, codes.found, codes.other, codes.temporary_moved)
 CONTENT_CHUNK_SIZE = 10 * 1024
 
@@ -493,7 +495,7 @@ class Request(object):
             if self.data:
 
                 body = self._encode_params(self.data)
-                if isinstance(self.data, str) or hasattr(self.data, 'read'):
+                if isinstance(self.data, str) or isinstance(self.data, builtin_str) or hasattr(self.data, 'read'):
                     content_type = None
                 else:
                     content_type = 'application/x-www-form-urlencoded'
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 10da6f7e..9204b15d 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -974,10 +974,10 @@ class RequestsTestSuite(TestSetup, TestBaseMixin, unittest.TestCase):
         self.assertEqual(t.get('files'), files)
 
     def test_str_data_content_type(self):
-        data = "test string data"
+        data = 'test string data'
         r = post(httpbin('post'), data=data)
         t = json.loads(r.text)
-        self.assertEqual(t.get('headers').get('Content-Type'), 'text/plain')
+        self.assertEqual(t.get('headers').get('Content-Type'), '')
 
 
 if __name__ == '__main__':