Handle posting same JSON twice.

Author: JingMa87

src/main/java/edu/harvard/iq/dataverse/LicenseServiceBean.java

@@ -55,6 +55,7 @@ public License getByName(String name) throws FetchException {
     public License save(License license) throws PersistenceException, RequestBodyException {
         if (license.getId() == null) {
             em.persist(license);
+            em.flush();
             return license;
         } else {
             throw new RequestBodyException("There shouldn't be an ID in the request body");