Api Library POST dont work after the change to use json_encode instead of http_build_query

[ghost]

Bug Description

Q	A
Mautic version	2.16.2
PHP version	7.3.17
Browser	Chrome

Steps to reproduce

POST using Mautic Api Library dont work while using OAuth2 (for instance to create contacts), The library was working fine with OAuth2 when the library was using http_build_query , but since the library started to use json_encode method the all functions that require POST stopped working while using OAuth but it still work fine with Basic Auth.
It return this error when try to create something
errors: { 0: { code: "401", message: "The response has unexpected status code (401)." }

Log errors

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[wise-gorilla]

@dongilbert @RCheesley I submitted a PR solving this issue, you can close this issue and merge this PR
#218