Remove manual serialization of payload. (#4525)

* Remove manual serialization of payload. 

Remove manual serialization of payload using JSON.stringify since it is default serializer.

* Update src/internal/observable/dom/webSocket.ts

Co-Authored-By: aerojeyenth <aerojeyenth@gmail.com>

* Update src/internal/observable/dom/webSocket.ts

Co-Authored-By: aerojeyenth <aerojeyenth@gmail.com>

* Update src/internal/observable/dom/webSocket.ts

Co-Authored-By: aerojeyenth <aerojeyenth@gmail.com>

src/internal/observable/dom/webSocket.ts

@@ -66,7 +66,7 @@ import { WebSocketSubject, WebSocketSubjectConfig } from './WebSocketSubject';
  * subscribes and unsubscribes. Server can use them to verify that some kind of messages should start or stop
  * being forwarded to the client. In case of the above example application, after getting subscription message with proper identifier,
  * gateway server can decide that it should connect to real sport news service and start forwarding messages from it.
- * Note that both messages will be sent as returned by the functions, meaning they will have to be serialized manually, just
+ * Note that both messages will be sent as returned by the functions, they are by default serialized using JSON.stringify, just
  * as messages pushed via `next`. Also bear in mind that these messages will be sent on *every* subscription and
  * unsubscription. This is potentially dangerous, because one consumer of an Observable may unsubscribe and the server
  * might stop sending messages, since it got unsubscription message. This needs to be handled
@@ -104,8 +104,8 @@ import { WebSocketSubject, WebSocketSubjectConfig } from './WebSocketSubject';
  * // Note that at least one consumer has to subscribe to the created subject - otherwise "nexted" values will be just buffered and not sent,
  * // since no connection was established!
  *
- * subject.next(JSON.stringify({message: 'some message'}));
- * // This will send a message to the server once a connection is made. Remember to serialize sent value first!
+ * subject.next({message: 'some message'});
+ * // This will send a message to the server once a connection is made. Remember value is serialized with JSON.stringify by default!
  *
  * subject.complete(); // Closes the connection.
  *
@@ -119,14 +119,14 @@ import { WebSocketSubject, WebSocketSubjectConfig } from './WebSocketSubject';
  * const subject = webSocket('ws://localhost:8081');
  *
  * const observableA = subject.multiplex(
- *   () => JSON.stringify({subscribe: 'A'}), // When server gets this message, it will start sending messages for 'A'...
- *   () => JSON.stringify({unsubscribe: 'A'}), // ...and when gets this one, it will stop.
+ *   () => ({subscribe: 'A'}), // When server gets this message, it will start sending messages for 'A'...
+ *   () => ({unsubscribe: 'A'}), // ...and when gets this one, it will stop.
  *   message => message.type === 'A' // If the function returns `true` message is passed down the stream. Skipped if the function returns false.
  * );
  *
  * const observableB = subject.multiplex( // And the same goes for 'B'.
- *   () => JSON.stringify({subscribe: 'B'}),
- *   () => JSON.stringify({unsubscribe: 'B'}),
+ *   () => ({subscribe: 'B'}),
+ *   () => ({unsubscribe: 'B'}),
  *   message => message.type === 'B'
  * );
  *