writerfilter: reset properties of reused styles

When we have a style to import and the name is already taken, we reuse
the style (we can't use replaceByName(), it only works with user-defined
styles). If that happens, reset non-default properties of it, so the
result will be what was in the file, not a merge of Writer defaults and
what was in the file.

Change-Id: Ifb1098f78254a061b11fd6c6d2a2fa8d85a00d60