MINIFICPP-1089 - Use after free in RESTSender::sendPayload

[arpadboda]

Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.

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 MINIFICPP-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:

• 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?
• If applicable, have you updated the NOTICE file?

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.

[bakaid]

Nice catch, we should really change the API to use shared_ptr-s in these cases, because this is the second such issue.
Do we have test coverage for these? If so, or you tested manually, I'm good with this.

[arpadboda]

@bakaid:

As far as I know it's covered by our unit test, but I also verify this on Unix and valgrind.

Created https://issues.apache.org/jira/browse/MINIFICPP-1090 to follow-up the issue in scope of 1.0.

[bakaid]

@arpadboda tested on macOS, seems to work fine, will merge soon.