Fix TR-1412 XML-encode values – CG

[wazelin]

TR-1412

• fix: use array element addition operand instead of calling array_push() function within qtism\data\storage\xml\Utils methods
• fix: XML-encode a value before inserting it into DOM

This is a legacy counterpart to #293

How to test

• author a test with Extended text interaction
• publish the test and launch the delivery as a test taker
• answer extended text interaction item with content containing non breaking spaces  , finish the test
• verify that result extraction consumer processed results without failure

[bugalot]

Sounds like an elegant solution!

[augustas]

• New code is covered by tests (if applicable)
• Tests are running successfully (old and new ones) on my local machine (if applicable)
• New code is respecting code style rules
• New code is respecting best practices
• New code is not subject to concurrency issues (if applicable)
• Feature is working correctly on my local machine (if applicable)
• Acceptance criteria are respected
• Pull request title and description are meaningful