update response header content-type

the previous type is default value, 'text/html'

cf-uaa-lib is attempting to talk to the CC, please see https://github.com/cloudfoundry/vcap-services/blob/master/oauth2/lib/service/provisioner.rb#L191 and https://github.com/cloudfoundry/cf-uaa-lib/blob/master/lib/uaa/http.rb#L79, the header content-type must be 'application/json'