Use system session in nashorn user mapper

2017-07-17 21:30:35,932 ERROR [http-bio-127.0.0.1-8080-exec-6] [nuxeo-error-log] org.nuxeo.ecm.core.api.NuxeoException: Cannot create a CoreSession outside a security context,  login() missing.
        at org.nuxeo.ecm.core.api.CoreInstance.getPrincipal(CoreInstance.java:220)
        at org.nuxeo.ecm.core.api.CoreInstance.openCoreSession(CoreInstance.java:71)
        at org.nuxeo.usermapper.extension.NashornUserMapper.resolveAttributes(NashornUserMapper.java:97)
        at org.nuxeo.usermapper.extension.AbstractUserMapper.getOrCreateAndUpdateNuxeoPrincipal(AbstractUserMapper.java:76)
        at org.nuxeo.usermapper.service.UserMapperComponent.getOrCreateAndUpdateNuxeoPrincipal(UserMapperComponent.java:112)
        at org.nuxeo.usermapper.service.UserMapperComponent.getOrCreateAndUpdateNuxeoPrincipal(UserMapperComponent.java:106)
        at org.nuxeo.ecm.platform.auth.saml.user.UserMapperBasedResolver.findOrCreateNuxeoUser(UserMapperBasedResolver.java:53)
        at org.nuxeo.ecm.platform.auth.saml.SAMLAuthenticationProvider.handleRetrieveIdentity(SAMLAuthenticationProvider.java:437)
        at org.nuxeo.ecm.platform.ui.web.auth.NuxeoAuthenticationFilter.handleRetrieveIdentity(NuxeoAuthenticationFilter.java:1022)