fast_mail_parser should return all headers

[bra-fsn]

Currently, fast_mail_parser return only the last header value if there are multiple headers with the same key (like Received, which is nearly almost the case).

An example program with Python's build-in parser:

# Hardcoded email data (for the sake of example, the email data is included here as a raw string)
email_data = """\
From: sender@example.com
To: recipient@example.com
Subject: Test Email
Date: Mon, 13 Sep 2024 10:00:00 +0200
Received: from mail.example.com (mail.example.com [192.0.2.1])
        by smtp.example.com with ESMTP id abc123
        for <recipient@example.com>; Mon, 13 Sep 2024 09:55:00 +0200
Received: from smtp2.example.com (smtp2.example.com [192.0.2.2])
        by mail.example.com with ESMTP id def456
        for <recipient@example.com>; Mon, 13 Sep 2024 09:50:00 +0200
Received: from relay.example.com (relay.example.com [192.0.2.3])
        by smtp2.example.com with ESMTP id ghi789
        for <recipient@example.com>; Mon, 13 Sep 2024 09:45:00 +0200
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit

This is a test email.
"""

import email
from email import policy
from email.parser import BytesParser
from io import BytesIO

def parse_received_headers(email_data):
    """Parse the Received: headers from a hardcoded EML string."""
    # Convert the email string to bytes (as we would normally read an EML file in bytes)
    email_bytes = BytesIO(email_data.encode('utf-8'))

    # Parse the email message
    email_message = BytesParser(policy=policy.default).parse(email_bytes)

    # Extract Received headers in the order they appear
    received_headers = []
    for header, value in email_message.items():
        if header.lower() == 'received':
            received_headers.append(value)

    return received_headers

# Call the function with the hardcoded email
received_headers = parse_received_headers(email_data)

# Print out the Received headers in order
for i, header in enumerate(received_headers, 1):
    print(f"Received Header {i}:\n{header}\n")

$ python test.py
Received Header 1:
from mail.example.com (mail.example.com [192.0.2.1])        by smtp.example.com with ESMTP id abc123        for <recipient@example.com>; Mon, 13 Sep 2024 09:55:00 +0200

Received Header 2:
from smtp2.example.com (smtp2.example.com [192.0.2.2])        by mail.example.com with ESMTP id def456        for <recipient@example.com>; Mon, 13 Sep 2024 09:50:00 +0200

Received Header 3:
from relay.example.com (relay.example.com [192.0.2.3])        by smtp2.example.com with ESMTP id ghi789        for <recipient@example.com>; Mon, 13 Sep 2024 09:45:00 +0200

and the same with fast_mail_parser:

email_data = """\
From: sender@example.com
To: recipient@example.com
Subject: Test Email
Date: Mon, 13 Sep 2024 10:00:00 +0200
Received: from mail.example.com (mail.example.com [192.0.2.1])
        by smtp.example.com with ESMTP id abc123
        for <recipient@example.com>; Mon, 13 Sep 2024 09:55:00 +0200
Received: from smtp2.example.com (smtp2.example.com [192.0.2.2])
        by mail.example.com with ESMTP id def456
        for <recipient@example.com>; Mon, 13 Sep 2024 09:50:00 +0200
Received: from relay.example.com (relay.example.com [192.0.2.3])
        by smtp2.example.com with ESMTP id ghi789
        for <recipient@example.com>; Mon, 13 Sep 2024 09:45:00 +0200
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit

This is a test email.
"""

from fast_mail_parser import parse_email, ParseError
from pprint import pprint as pp
email = parse_email(email_data)
pp(email.headers)

$ python test-fmp.py
{'Content-Transfer-Encoding': '7bit',
 'Content-Type': 'text/plain; charset="utf-8"',
 'Date': 'Mon, 13 Sep 2024 10:00:00 +0200',
 'From': 'sender@example.com',
 'MIME-Version': '1.0',
 'Received': 'from relay.example.com (relay.example.com [192.0.2.3]) by '
             'smtp2.example.com with ESMTP id ghi789 for '
             '<recipient@example.com>; Mon, 13 Sep 2024 09:45:00 +0200',
 'Subject': 'Test Email',
 'To': 'recipient@example.com'}

Specifically for the Received header, the most significant one if the first, but fast_mail_parser returns only the last one. Could you please add (not to break compatibility) another header representation, which correctly lists all headers, maybe with a list of tuples?