Updated the batch secrets retrieval header param

[telday]

What does this PR do?

The header parameter used to set secrets to base64 encode
on batch retrieval was changed from Accept to Accept-Encoding
in Conjur, this change reflects that.

Resolves #99

Checklists

Change log

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• This PR does not require updating any documentation

[izgeri]

this needs a changelog message, since we changed the header type. we should be clear the issue or pull that this is associated with in Conjur, too