fix keep alive data back

Author: whyoleg

rsocket-core/src/commonMain/kotlin/io/rsocket/kotlin/connection/KeepAliveHandler.kt

@@ -17,6 +17,7 @@
 package io.rsocket.kotlin.connection
 
 import io.rsocket.kotlin.*
+import io.rsocket.kotlin.frame.io.*
 import io.rsocket.kotlin.keepalive.*
 import kotlinx.atomicfu.*
 import kotlinx.coroutines.*
@@ -41,9 +42,7 @@ internal class KeepAliveHandler(
                 if (currentDelayMillis() - lastMark.value >= keepAlive.maxLifetimeMillis)
                     throw RSocketError.ConnectionError("No keep-alive for ${keepAlive.maxLifetimeMillis} ms")
 
-                outbound.sendKeepAlive(true, Buffer().apply {
-                    writeString(currentDelayMillis().toString())
-                }, 0)
+                outbound.sendKeepAlive(true, EmptyBuffer, 0)
             }
         }
     }