Enable SocketKeepalive with 15s by default (matches Golang's default)

cluster.go

@@ -117,7 +117,7 @@ type ClusterConfig struct {
 	// Default reconnection policy to use for reconnecting before trying to mark host as down.
 	ReconnectionPolicy ReconnectionPolicy
 
-	// The keepalive period to use, enabled if > 0 (default: 0)
+	// The keepalive period to use, enabled if > 0 (default: 15 seconds)
 	// SocketKeepalive is used to set up the default dialer and is ignored if Dialer or HostDialer is provided.
 	SocketKeepalive time.Duration
 
@@ -290,6 +290,7 @@ func NewCluster(hosts ...string) *ClusterConfig {
 		ReconnectInterval:      60 * time.Second,
 		ConvictionPolicy:       &SimpleConvictionPolicy{},
 		ReconnectionPolicy:     &ConstantReconnectionPolicy{MaxRetries: 3, Interval: 1 * time.Second},
+		SocketKeepalive:        15 * time.Second,
 		WriteCoalesceWaitTime:  200 * time.Microsecond,
 	}