OUString::trim() uses rtl_uString_newTrim, which relies upon
rtl_ImplIsWhitespace. The latter treats as whitespaces not only
characters with values less than or equal to 32, but also Unicode
General Punctuation area Space and some Control characters. Thus,
using OUString::trim() is incorrect when the goal is to trim XML
whitespace, which is defined as one of 0x09, 0x0A, 0x0D, 0x20.

The comments for OUString::trim() and rtl_uString_newTrim are
corrected to describe which characters are considered whitespace.

A unit test included.

Change-Id: I45a132be923a52dcd5a4c35aeecb53d423b49fec
Reviewed-on: https://gerrit.libreoffice.org/41444Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>