fix: Avoid pickling processed credentials (#1016)

Author: andrewsg

google/cloud/storage/client.py

@@ -130,10 +130,11 @@ def __init__(
  if project is _marker:
  project = None
 
- # Save the initial value of client_info and client_options before they
+ # Save the initial value of constructor arguments before they
  # are passed along, for use in __reduce__ defined elsewhere.
  self._initial_client_info = client_info
  self._initial_client_options = client_options
+ self._initial_credentials = credentials
 
  kw_args = {"client_info": client_info}
 

google/cloud/storage/transfer_manager.py

@@ -879,7 +879,7 @@ def _reduce_client(cl):
 
  client_object_id = id(cl)
  project = cl.project
- credentials = cl._credentials
+ credentials = cl._initial_credentials
  _http = None # Can't carry this over
  client_info = cl._initial_client_info
  client_options = cl._initial_client_options