Unload root element if Part.GetStream updates the underlying value

[twsouthwick]

Fixes #1755

[github-actions]

Test Results

    70 files  ± 0      70 suites  ±0   57m 52s ⏱️ +56s
 2 032 tests + 4   2 029 ✅ + 4   3 💤 ±0  0 ❌ ±0 
32 310 runs  +48  32 274 ✅ +48  36 💤 ±0  0 ❌ ±0 

Results for commit cf89336. ± Comparison against base commit cd814ec.

[benjaminki]

I'm new to reviewing OpenXml source, but I reviewed these changes and they look good to me. I think the issue I was encountering is related to DOM and SAX used together; I saw my SAX changes not being preserved and I think its because the DOM objects in memory were overwriting them when saving to disk; with these changes I see unloading of DOM object being queued whenever stream is written to. The Unit Test GetStreamWrite looks like a repro of the issue I was facing, so these changes should fix what I was experiencing.