Parameter update for post method in http client interface

[simpleadm]

Updated $params parameter for post method of \Magento\Framework\HTTP\ClientInterface interface.

Description

Currently all classes that implement \Magento\Framework\HTTP\ClientInterface interface already have (or will have soon in all branches) support of string in post method. So current PR populate that useful ability to the interface.

Related issue and PRs

• CURL Json POST CURL Json POST #3489
• Socket Json POST Add json and xml support to the post method in socket client #14169

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @larsroettig, thank you for the review.
ENGCOM-1199 has been created to process this Pull Request