XSLX2CSV: Do not double-encode if the value is already having quotes and escape doubl...

XSLX2CSV: Do not double-encode if the value is already having quotes and escape double-quotes

Most CSV formats use "" (two quotes) to escape a "-character, we should do this in this
example as well to produce files that can be parsed by other CSV processors correctly.

Also cases where the value is already enclosed in quotes should not lead to additional quotes

Add a simple initial test to module "examples" verify basic functionality of XSLX2CSV
as I often rely on it for converting some very large xlsx-files to csv

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1888418 13f79535-47bb-0310-9956-ffa450edef68