Fixed system tags DAV and API and docs

apps/dav/lib/systemtag/systemtagnode.php

apps/dav/lib/systemtag/systemtagplugin.php

apps/dav/lib/systemtag/systemtagsbyidcollection.php

apps/dav/lib/systemtag/systemtagsobjectmappingcollection.php

apps/dav/lib/systemtag/systemtagsobjecttypecollection.php

apps/dav/tests/unit/systemtag/systemtagsbyidcollection.php

...ests/unit/systemtag/systemtagsobjectmappingcollection.php

lib/private/systemtag/systemtagmanager.php

lib/private/systemtag/systemtagobjectmapper.php

lib/public/systemtag/isystemtagmanager.php

tests/lib/systemtag/systemtagmanagertest.php

tests/lib/systemtag/systemtagobjectmappertest.php