v2: Content-Type: application/octet-stream header always added

The bug: any existing Content-Type header cannot be found because the
call to headers.get() fails. Therefore we end up with two Content-Type
headers because a new one (applicaion/octet-stream) gets added
unconditionally. The cause: the strings (keys and values) in the headers
dict are converted from unicode sequences of type <str> to utf-8
sequences of type <bytes>. This happens in safe_encode()
(oslo_utils/encodeutils.py:66). <str> != <bytes> even if they appear to
have the same characters.

Hence, for python 3.x, _set_common_request_kwargs() adds content-type
to header even if custom content-type is set in the request.

This results in unsupported media type exception when glance client
is used with keystoneauth and python 3.x

The fix: follow the directions in encode_headers().
It says to do this just before sending the request. Honor this principle;
do not encode headers and then perform more business logic on them.

Change-Id: Idf6079b32f70bc171f5016467048e917d42f296d
Closes-bug: #1641239
Co-Authored-By: Pushkar Umaranikar <pushkar.umaranikar@intel.com>

glanceclient/common/http.py

@@ -315,16 +315,18 @@ class SessionClient(adapter.Adapter, _BaseHTTPClient):
         super(SessionClient, self).__init__(session, **kwargs)
 
     def request(self, url, method, **kwargs):
-        headers = encode_headers(kwargs.pop('headers', {}))
+        headers = kwargs.pop('headers', {})
         kwargs['raise_exc'] = False
         data = self._set_common_request_kwargs(headers, kwargs)
-
         try:
-            resp = super(SessionClient, self).request(url,
-                                                      method,
-                                                      headers=headers,
-                                                      data=data,
-                                                      **kwargs)
+            # NOTE(pumaranikar): To avoid bug #1641239, no modification of
+            # headers should be allowed after encode_headers() is called.
+            resp = super(SessionClient,
+                         self).request(url,
+                                       method,
+                                       headers=encode_headers(headers),
+                                       data=data,
+                                       **kwargs)
         except ksa_exc.ConnectTimeout as e:
             conn_url = self.get_endpoint(auth=kwargs.get('auth'))
             conn_url = "%s/%s" % (conn_url.rstrip('/'), url.lstrip('/'))