Prevent changing content type in request

Now if body contains json, its content type forcibly
changes to application/json, which is not correct in
some case, like json-patch request.

This code fixes this situation and sets application/json
contnent type only if this header hasn't been set before.

Change-Id: I4e0c44d444519f056dfa48c9603dbc3ca6b01822
Closes-bug: #1634110

keystoneauth1/session.py

@@ -543,7 +543,7 @@ class Session(object):
                                'for=%s;by=%s' % (self.original_ip, user_agent))
 
         if json is not None:
-            headers['Content-Type'] = 'application/json'
+            headers.setdefault('Content-Type', 'application/json')
             kwargs['data'] = self._json.encode(json)
 
         for k, v in self.additional_headers.items():

keystoneauth1/tests/unit/test_session.py

@@ -284,6 +284,25 @@ class SessionTests(utils.TestCase):
                                session.get,
                                self.TEST_URL)
 
+    def test_json_content_type(self):
+        session = client_session.Session()
+        self.stub_url('POST', text='response')
+        resp = session.post(
+            self.TEST_URL,
+            json=[{'op': 'replace',
+                   'path': '/name',
+                   'value': 'new_name'}],
+            headers={'Content-Type': 'application/json-patch+json'})
+
+        self.assertEqual('POST', self.requests_mock.last_request.method)
+        self.assertEqual(resp.text, 'response')
+        self.assertTrue(resp.ok)
+        self.assertRequestBodyIs(
+            json=[{'op': 'replace',
+                   'path': '/name',
+                   'value': 'new_name'}])
+        self.assertContentTypeIs('application/json-patch+json')
+
 
 class RedirectTests(utils.TestCase):
 

keystoneauth1/tests/unit/utils.py

@@ -53,7 +53,7 @@ class TestCase(testtools.TestCase):
         if json:
             kwargs['text'] = jsonutils.dumps(json)
             headers = kwargs.setdefault('headers', {})
-            headers['Content-Type'] = 'application/json'
+            headers.setdefault('Content-Type', 'application/json')
 
         if parts:
             url = '/'.join([p.strip('/') for p in [base_url] + parts])
@@ -71,6 +71,10 @@ class TestCase(testtools.TestCase):
         elif body:
             self.assertEqual(body, last_request_body)
 
+    def assertContentTypeIs(self, content_type):
+        last_request = self.requests_mock.last_request
+        self.assertEqual(last_request.headers['Content-Type'], content_type)
+
     def assertQueryStringIs(self, qs=''):
         r"""Verify the QueryString matches what is expected.