Fix innerHTML/innerText to coerce non-string values

[dyk1454683243-sudo]

Fixes #308

Setting innerHTML or innerText with a non-string (for example 1 or null) currently throws. Browsers treat these accessors as [LegacyNullToEmptyString] DOMString: null becomes '', and any other value is stringified.

This keeps the change on the setter path only:

• setInnerHtml coerces with html === null ? '' : String(html) before parseFromString
• Element now has a matching set innerText that uses the same coerce and replaces children like textContent (the existing get innerText whitespace rules are unchanged)
• Regression tests cover innerHTML = 1, innerHTML = null, and innerText = 1

document.body.innerHTML = 1;    // '1', no throw
document.body.innerHTML = null; // ''
document.body.innerText = 1;    // visible text '1'