Update pipeline_client.py

fix string_data to str_data

Author: felixhjh

python/pipeline/pipeline_client.py

@@ -108,7 +108,7 @@ def _pack_request_package(self, feed_dict, pack_tensor_format,
                 one_tensor.name = key
 
                 if isinstance(value, str):
-                    one_tensor.string_data.add(value)
+                    one_tensor.str_data.append(value)
                     one_tensor.elem_type = 12  #12 => string in proto
                     continue