More teImprove test coverage.

src/main/java/com/zaxxer/hikari/proxy/ClosedConnection.java

@@ -38,10 +38,7 @@ final class ClosedConnection
          public Object invoke(Object proxy, Method method, Object[] args) throws Throwable
          {
             final String methodName = method.getName();
-            if ("isClosed".equals(methodName)) {
+            if ("close".equals(methodName)) {
-               return Boolean.TRUE;
-            }
-            else if ("close".equals(methodName)) {
                return Void.TYPE;
             }
             else if ("abort".equals(methodName)) { 

src/test/java/com/zaxxer/hikari/TestConnections.java

@@ -203,7 +203,12 @@ public class TestConnections
       try {
          Connection connection = ds.getConnection();
          connection.close();
+
+         // should no-op
+         connection.abort(null);
+
          Assert.assertTrue("Connection should have closed", connection.isClosed());
+         Assert.assertFalse("Connection should have closed", connection.isValid(5));
          Assert.assertTrue("Expected to contain ClosedConnection, but was " + connection.toString(), connection.toString().contains("ClosedConnection"));
 
          connection.close();