Enable SP to retrieve and provide German umlaut chars

[john-odonnell]

Desired Outcome

Enable Secrets Provider to deliver secrets with special characters/binary values.

Implemented Changes

In conjur#1989, the /secrets endpoint was updated to allow for the batch retrieval of secrets with binary values - adding the Accept-Encoding header set to base64 means Conjur will base64 encode secret values before attempting to render response JSON.

We can use that functionality here to enable Secrets Provider to deliver binary/special character secrets by switching client.RetrieveBatchSecrets() to client.RetrieveBatchSecretsSafe().

Connected Issue/Story

CNJR-466

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be merged.

Changelog

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

Test coverage

• [x] 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
• [ ] A follow-up issue to update official docs has been filed here: [insert issue ID]
• [x] This PR does not require updating any documentation

Behavior

• [ ] This PR changes product behavior and has been reviewed by a PO, or
• [ ] These changes are part of a larger initiative that will be reviewed later, or
• [x] No behavior was changed with this PR

Security

• [ ] Security architect has reviewed the changes in this PR,
• [ ] These changes are part of a larger initiative with a separate security review, or
• [x] There are no security aspects to these changes

[codeclimate[bot]]

Code Climate has analyzed commit 9c027b92 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 89.1% (0.0% change).

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