BUGFIX Allowing TimeField and subclasses to be set to NULL through setValue()
BUGFIX Added TimeField->dataValue() to apply same conversion as setValue()
MINOR Added TimeFieldTest
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@79355 467b73ca-7a2a-4603-9d3b-597d59a354a9