Fix query params type

[dtaniwaki]

I got the following error when I tried IamAwsProvider because there's a typo.

>>> from minio.credentials import IamAwsProvider
>>>
>>> cred = IamAwsProvider()
>>> cred.retrieve()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/src/minio/minio/credentials/providers.py", line 432, in retrieve
    self._credentials = provider.retrieve()
  File "/usr/local/src/minio/minio/credentials/providers.py", line 576, in retrieve
    query_params["Action"] = "AssumeRoleWithWebIdentity"
TypeError: 'set' object does not support item assignment

[balamurugana]

Fixed by #1000