rest: handle endpoints that don't output json

The /changes/<id>/revisions/current/patch?zip endpoint doesn't
return json output but a binary data (the zipped contents of the
change). The usual get() function raises a ValueError when
faced with this. As a workaround provide a get_raw() function
to allow callers to handle the decoding of data.

pygerrit/rest/__init__.py

@@ -118,6 +118,25 @@ def get(self, endpoint, **kwargs):
         response = requests.get(self.make_url(endpoint), **kwargs)
         return _decode_response(response)
 
+    def get_raw(self, endpoint, **kwargs):
+        """ Send HTTP GET to the endpoint. Return raw result
+
+        :arg str endpoint: The endpoint to send to.
+
+        :returns:
+            Raw result.
+
+        :raises:
+            requests.RequestException on timeout or connection error.
+
+        """
+        kwargs.update(self.kwargs.copy())
+        response = requests.get(self.make_url(endpoint), **kwargs)
+        content = response.content.strip()
+        response.raise_for_status()
+
+        return content
+
     def put(self, endpoint, **kwargs):
         """ Send HTTP PUT to the endpoint.