A CharSequence has to return its content in toString().

Change-Id: I070af386ee61bb6fc4193da4c08c1a541b39b51b
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/185264
Reviewed-by: Jörg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>
Tested-by: JDT Bot <jdt-bot@eclipse.org>