[jwt] Fix response for pubilc key server request

[wing2fly]

What changes were proposed in this pull request?

Fix the response from the call to public keys server and loads the json content from response.

How was this patch tested?

Tested in real environment with Hue shell, and fix the unit test.

[root@hue]# build/env/bin/hue shell

(InteractiveConsole)
>>> 
>>> 
>>> import json
>>> import requests
>>> import jwt
>>> from cryptography.hazmat.primitives import serialization
>>> 
>>> headers = {'kid': '1'} 
>>> response = requests.get('http://public_key_server:1234/', headers=headers)
>>> jwk = json.loads(response.content)
>>> public_key = jwt.algorithms.RSAAlgorithm.from_jwk(json.dumps(jwk["keys"][0])).public_key()
>>> public_key_pem = public_key.public_bytes(encoding=serialization.Encoding.PEM, format=serialization.PublicFormat.SubjectPublicKeyInfo)
>>> public_key_pem
'-----BEGIN PUBLIC KEY-----\
.....
-----END PUBLIC KEY-----\n'
>>> 

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