Fix character encoding when inserting Form after specific HTML element

[n7studios]

Summary

Fixes this reported character encoding issue by forcibly loading the Post / Page content with the UTF-8 character set.

Before:

After:

Testing

• testAddNewPageUsingDefaultFormAfterHeadingElement: Added missing test to assert Form is correctly positioned after a heading on Pages.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)