[sitecore-jss-nexts] Consider existing CORS header when running enforceCors

[art-alexeyenko]

• [x] This PR follows the Contribution Guide
• [x] Changelog updated
• [ ] Upgrade guide entry added

Description / Motivation

Followup to https://github.com/Sitecore/jss/pull/1798 EnforceCors function should also check the previously set CORS headers if present, as editing middleware will be requested from sitecoreApiHost too.

Testing Details

• [x] Unit Test Added
• [ ] Manual Test/Other (Please elaborate)

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)