Revert 3ff3ed0c56 case-insensitive compares.

My oops.  The comparisons, which are copied from the IMipPlugin shipped with sabre-io/dav,
do not need to be case insensitive because the sender and recipient names are normalized by sabre,
(see calls to getNormalizedValue in voboject/lib/ITip/Broker.php).

Signed-off-by: Brad Rubenstein <brad@wbr.tech>