fix(fetch): match Content-Type header case-insensitively

[Alexandre Kohler (kwy404)]

is_json_content_type only matched the exact header name Content-Type, so a request sent with content-type: application/json had its string data passed through unchanged instead of being treated as JSON. Header names are case-insensitive and Playwright for JS already compares them that way, so the check now lowercases the name.

The lowercase header case is covered by the new test_should_json_stringify_string_body_when_content_type_is_application_json in tests/async/test_fetch_global.py.