NIFI-5937 use processor-configured encoding instead of the system default

[SavtechSolutions]

Thank you for submitting a contribution to Apache NiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• Have you written or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[bdesert]

Reviewing

[SavtechSolutions]

added some unit tests (copied them from my other PR that got stuck in review for whatever reason)

[bdesert]

Checkstyle is failing:
[WARNING] src/test/java/org/apache/nifi/processors/elasticsearch/TestPutElasticsearchHttpRecord.java:[20] (imports) AvoidStarImport: Using the '.' form of import should be avoided - okhttp3..
[WARNING] src/test/java/org/apache/nifi/processors/elasticsearch/TestPutElasticsearchHttpRecord.java:[43] (imports) AvoidStarImport: Using the '.' form of import should be avoided - org.junit.Assert..

I would recommend to run
mvn -T 2 clean install -Pcontrib-check -Ddir-only
on the projects that have been changed.

[bdesert]

+1 LGTM. Ran unit tests and with elastic-mock. works as expected.Thanks for the improvement! Merged to master