fix: Fix serialization payload in Request. Fix Docs for Post Request (#683)

### Description

An incorrect serialization for `payload` is happening now. `payload` is
represented as a byte string, but bytes cannot be serialized to json.
Therefore, when loading and unloading from the queue
(`_persist_single_request_to_storage`, `_json_to_request`) we get
strings of format “b'test'”

Also corrected the documentation, as when filling forms the data should
be passed not in json format and have the appropriate header.

### Issues

- Closes: #668 

### Testing

Updated `test_request_state_serialization` to also take `payload` into
account when serializing and deserializing `Request`

### Checklist

- [x] CI passed

---------

Co-authored-by: Vlada Dusek <v.dusek96@gmail.com>

Author: Mantisus

docs/examples/code/fill_and_submit_web_form_crawler.py

@@ -1,5 +1,5 @@
 import asyncio
-import json
+from urllib.parse import urlencode
 
 from crawlee import Request
 from crawlee.http_crawler import HttpCrawler, HttpCrawlingContext
@@ -19,7 +19,8 @@ async def request_handler(context: HttpCrawlingContext) -> None:
     request = Request.from_url(
         url='https://httpbin.org/post',
         method='POST',
-        payload=json.dumps(
+        headers={'content-type': 'application/x-www-form-urlencoded'},
+        payload=urlencode(
             {
                 'custname': 'John Doe',
                 'custtel': '1234567890',

docs/examples/code/fill_and_submit_web_form_request.py

@@ -1,12 +1,13 @@
-import json
+from urllib.parse import urlencode
 
 from crawlee import Request
 
 # Prepare a POST request to the form endpoint.
 request = Request.from_url(
     url='https://httpbin.org/post',
     method='POST',
-    payload=json.dumps(
+    headers={'content-type': 'application/x-www-form-urlencoded'},
+    payload=urlencode(
         {
             'custname': 'John Doe',
             'custtel': '1234567890',

src/crawlee/_request.py

@@ -8,15 +8,7 @@
 from enum import IntEnum
 from typing import Annotated, Any, cast
 
-from pydantic import (
-    BaseModel,
-    BeforeValidator,
-    ConfigDict,
-    Field,
-    PlainSerializer,
-    PlainValidator,
-    TypeAdapter,
-)
+from pydantic import BaseModel, BeforeValidator, ConfigDict, Field, PlainSerializer, PlainValidator, TypeAdapter
 from typing_extensions import Self
 
 from crawlee._types import EnqueueStrategy, HttpHeaders, HttpMethod, HttpPayload, JsonSerializable
@@ -143,7 +135,11 @@ class BaseRequestData(BaseModel):
     headers: Annotated[HttpHeaders, Field(default_factory=HttpHeaders)] = HttpHeaders()
     """HTTP request headers."""
 
-    payload: HttpPayload | None = None
+    payload: Annotated[
+        HttpPayload | None,
+        BeforeValidator(lambda v: v.encode() if isinstance(v, str) else v),
+        PlainSerializer(lambda v: v.decode() if isinstance(v, bytes) else None),
+    ] = None
     """HTTP request payload."""
 
     user_data: Annotated[

tests/unit/_memory_storage_client/test_request_queue_client.py

@@ -93,19 +93,19 @@ async def test_list_head(request_queue_client: RequestQueueClient) -> None:
 
 
 async def test_request_state_serialization(request_queue_client: RequestQueueClient) -> None:
-    request = Request.from_url('https://crawlee.dev')
+    request = Request.from_url('https://crawlee.dev', payload=b'test')
     request.state = RequestState.UNPROCESSED
 
     await request_queue_client.add_request(request)
 
     result = await request_queue_client.list_head()
-
     assert len(result.items) == 1
     assert result.items[0] == request
 
     got_request = await request_queue_client.get_request(request.id)
 
     assert request == got_request
+    assert request.payload == got_request.payload
 
 
 async def test_add_record(request_queue_client: RequestQueueClient) -> None: