heartbeat: ignore "Connection has been closed" error

It can happen that connection is being closed between `isClosed()` from
userEventTriggered `isClosed()` from `write()`.
Which will result in netty event loop printing warning.
Not a big deal, but it confuses QA in regards of the analyzing if this
is a real error or not.
So, let's just ignore it.

Author: dkropachev

driver-core/src/main/java/com/datastax/driver/core/Connection.java

@@ -1592,7 +1592,13 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exc
             "{} was inactive for {} seconds, sending heartbeat",
             Connection.this,
             factory.configuration.getPoolingOptions().getHeartbeatIntervalSeconds());
-        write(HEARTBEAT_CALLBACK);
+        try {
+          write(HEARTBEAT_CALLBACK);
+        } catch (ConnectionException e) {
+          if (!e.getMessage().contains("Connection has been closed")) {
+            throw e;
+          }
+        }
       }
     }