Fix case sensitivity in minio metadata

[hardbyte]

Field names in HTTP headers are case-insensitive, some networks decide that means that can normalize the headers however they like, presumably to keep things interesting.

On one such network the backend errors with the following:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/var/www/entityservice/tasks/base_task.py", line 87, in __call__
    return super(TracedTask, self).__call__(*args, **kwargs)
  File "/usr/lib/python3.8/site-packages/celery/app/trace.py", line 650, in __protected_call__
    return self.run(*args, **kwargs)
  File "/var/www/entityservice/tasks/encoding_uploading.py", line 72, in pull_external_data
    count = int(stat.metadata['X-Amz-Meta-Hash-Count'])
KeyError: 'X-Amz-Meta-Hash-Count'

Minio's stat.metadata is a dict of custom HTTP headers used to transfer metadata from the anonlink-client to the server via the object store. This small change ensures that the server will get the correct values regardless of the case of the header.

[joyceyuu]

Look good to me!