rest_dbus: run Json format plugins conditionally
rest_dbus: run Json format plugins conditionally

Run Json format verification plug-ins only if the content-type is
'application/json'.

Change-Id: Ia6e089b94cc24fbd630042414335af7ce1ccd707
Signed-off-by: Deepak Kodihalli <54105a30055f1e07853b8ee7a2f0d75d22ed98c1@in.ibm.com>

openbmc/phosphor-rest-server,openbmc/phosphor-rest-server