Add test for beginTransaction() with existing active transaction

src/test/java/com/avaje/tests/transaction/TestBeginTransactionWithExisting.java

@@ -0,0 +1,24 @@
+package com.avaje.tests.transaction;
+
+import com.avaje.ebean.BaseTestCase;
+import com.avaje.ebean.Ebean;
+import com.avaje.ebean.Transaction;
+import org.junit.Test;
+
+import javax.persistence.PersistenceException;
+
+public class TestBeginTransactionWithExisting extends BaseTestCase {
+
+  @Test(expected = PersistenceException.class)
+  public void test() {
+
+    Transaction txn = Ebean.beginTransaction();
+    try {
+
+      Ebean.beginTransaction();
+
+    } finally {
+      txn.end();
+    }
+  }
+}