fix: update connection.ts to always clear the heartbeat interval (#1942)

Co-authored-by: Jackson Mills <jmills@ripple.com>
Co-authored-by: Elliot Lee <github.public@intelliot.com>

packages/xrpl/src/client/connection.ts

@@ -273,6 +273,7 @@ export class Connection extends EventEmitter {
    * @returns A promise containing either `undefined` or a disconnected code, that resolves when the connection is destroyed.
    */
   public async disconnect(): Promise<number | undefined> {
+    this.clearHeartbeatInterval()
     if (this.reconnectTimeoutID !== null) {
       clearTimeout(this.reconnectTimeoutID)
       this.reconnectTimeoutID = null