[azfs] support utf-8 characters in the azure key.

[yongtang]

This is from https://github.com/tensorflow/io/issues/500#issuecomment-581509214 @niklas-mohr-jdas

Does the above mentioned PR also fix the issue with utf-8 characters in the azure key?

import os
import tensorflow as tf
import tensorflow_io as tfio
os.environ['TF_AZURE_STORAGE_KEY']="Bär"
tf.io.gfile.exists("az://account/container/test.txt")

triggers an error

terminate called after throwing an instance of 'std::runtime_error'
  what():  length of base64 string is not an even multiple of 4
Aborted

using tensorflow==2.1.0 and tensorflow_io==0.11.0. The same code works with a key os.environ['TF_AZURE_STORAGE_KEY']="Bur"

[janbernloehr]

Does this issue still exist now that the fix in tf was merged?