getMarkRange() will not return the actual end position of the current mark.
might reintroduce bug from #156 (i cannot reproduce #156 so i cannot check)
fixes in this commit:
A) work around a possible bug in indexAfter()?:
$pos.indexAfter() seems to return the same index as $pos.index() at some point
-> fixed by increasing startIndex by 1 instead of using indexAfter()
B) endPos needs to be initialized with initial startPos + nodeSize and not
with resulting startPos:
-> moved initialization right after setting startPos
