[Bug]: Request.post_data_json raises KeyError when content-type is missing #3093

Version

main branch source as of 2026-06-08 (latest release is 1.60.0)

Steps to reproduce

Request.post_data_json currently assumes every request with a post body has a content-type header.

A minimal source-level reproduction against the current repo:

import base64
import sys
import types

repo_version = types.ModuleType("playwright._repo_version")
repo_version.version = "0.0.0-test"
sys.modules["playwright._repo_version"] = repo_version

from playwright._impl._network import RawHeaders, Request, SerializedFallbackOverrides

req = Request.__new__(Request)
req._initializer = {
    "url": "https://example.test/",
    "method": "POST",
    "resourceType": "xhr",
    "headers": [],
    "postData": base64.b64encode(b'{"a": 1}').decode(),
}
req._fallback_overrides = SerializedFallbackOverrides()
req._provisional_headers = RawHeaders(req._initializer["headers"])

print(req.post_data_json)

Expected behavior

request.post_data_json should treat the missing content-type header as "not form-urlencoded" and then use the documented JSON fallback. For the body above, it should return {"a": 1}.

This matches the upstream JavaScript implementation, which checks the header with optional access before the form-urlencoded branch.

Actual behavior

The property raises before it reaches JSON parsing:

KeyError: 'content-type'

The source currently does this:

content_type = self.headers["content-type"]
if "application/x-www-form-urlencoded" in content_type:
    ...

Additional context

The API docs say Request.post_data_json returns parsed form-urlencoded data and parses JSON as a fallback. A missing content-type header should not prevent that fallback path.