Bug 51158: clear out Memory-based part before writing to it,
except for PictureData items, which keep the original image
data in the Part-object directly.
Add reproducer-unit tests and enhance some related unit tests

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1553525 13f79535-47bb-0310-9956-ffa450edef68