Tilee/fix argoutofrange

[TimothyMothra]

Fix Issue #1513

Fixing ArgumentOutOfRangeException.
W3CUtilities could have thrown if IndexOf returns a -1.

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

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• CHANGELOG.md updated with one line description of the fix, and a link to the original issue.

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