Fix modification of live proxy

Two kinds of modification lead to an error:

• update a document with version increment

  Caused by: org.nuxeo.ecm.core.api.DocumentException: No schema for prefix: ecm:minorVersion
  	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentProxy.getSchema(SQLDocumentProxy.java:65)
  	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentProxy.isPropertyForProxy(SQLDocumentProxy.java:83)
  	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentProxy.getPropertyValue(SQLDocumentProxy.java:438)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.getVersion(StandardVersioningService.java:113)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.getMinor(StandardVersioningService.java:108)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.incrementMinor(StandardVersioningService.java:132)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.incrementByOption(StandardVersioningService.java:141)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.doPostSave(StandardVersioningService.java:309)
  	at org.nuxeo.ecm.core.versioning.VersioningComponent.doPostSave(VersioningComponent.java:225)
  	at org.nuxeo.ecm.core.api.AbstractSession.saveDocument(AbstractSession.java:1708)
  

• update of a checkin document

  Caused by: java.lang.UnsupportedOperationException
  	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentProxy.checkOut(SQLDocumentProxy.java:303)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.doCheckOut(StandardVersioningService.java:326)
  	at org.nuxeo.ecm.core.versioning.StandardVersioningService.doPreSave(StandardVersioningService.java:276)
  	at org.nuxeo.ecm.core.versioning.VersioningComponent.doPreSave(VersioningComponent.java:211)
  	at org.nuxeo.ecm.core.api.AbstractSession.saveDocument(AbstractSession.java:1676)
  	at sun.reflect.GeneratedMethodAccessor630.invoke(Unknown Source)
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  	at java.lang.reflect.Method.invoke(Method.java:606)
  	at org.nuxeo.ecm.core.api.TransactionalCoreSessionWrapper.invoke(TransactionalCoreSessionWrapper.java:131)
  	at com.sun.proxy.$Proxy64.saveDocument(Unknown Source)
  	at org.nuxeo.ecm.webapp.contentbrowser.DocumentActionsBean.updateDocument(DocumentActionsBean.java:286)