OAuth2: fix “Nested Transactions are not supported” when accessing a document with a bearer token

javax.transaction.NotSupportedException: Nested Transactions are not supported
	at org.apache.geronimo.transaction.manager.TransactionManagerImpl.begin(TransactionManagerImpl.java:157)
	at org.apache.geronimo.transaction.manager.TransactionManagerImpl.begin(TransactionManagerImpl.java:152)
	at org.nuxeo.runtime.jtajca.NuxeoContainer$UserTransactionImpl.begin(NuxeoContainer.java:522)
	at org.nuxeo.runtime.transaction.TransactionHelper.startTransaction(TransactionHelper.java:241)
	at org.nuxeo.runtime.transaction.TransactionHelper.startTransaction(TransactionHelper.java:335)
	at org.nuxeo.ecm.platform.web.common.ServletHelper.startTransaction(ServletHelper.java:57)
	at org.nuxeo.ecm.platform.web.common.requestcontroller.filter.NuxeoRequestControllerFilter.doFilter(NuxeoRequestControllerFilter.java:168)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
	at org.nuxeo.ecm.platform.ui.web.auth.NuxeoAuthenticationFilter.doFilterInternal(NuxeoAuthenticationFilter.java:437)
	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:51)
	at org.nuxeo.ecm.platform.ui.web.auth.oauth.NuxeoOAuthFilter.doFilter(NuxeoOAuthFilter.java:122)
	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:49)
	at org.nuxeo.ecm.platform.ui.web.auth.oauth2.NuxeoOAuth2Filter.processAuthentication(NuxeoOAuth2Filter.java:158)
	at org.nuxeo.ecm.platform.ui.web.auth.oauth2.NuxeoOAuth2Filter.process(NuxeoOAuth2Filter.java:132)
	at org.nuxeo.ecm.platform.ui.web.auth.oauth2.NuxeoOAuth2Filter.doFilter(NuxeoOAuth2Filter.java:93)
	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:49)
	at org.nuxeo.ecm.platform.ui.web.auth.NuxeoAuthenticationFilter.doFilter(NuxeoAuthenticationFilter.java:409)