Cheroot clobbers previous lines' values during field line folding

[kenballus]

❓ I'm submitting a ...

• 🐞 bug report
• 🐣 feature request
• ❓ question about the decisions made in the repository

🐞 Describe the bug. What is the current behavior?
When Cheroot receives a request that uses field line folding, it uses the last line's value instead of concatenating all received lines' values together.

i.e., when Cheroot receives a request like this:

GET / HTTP/1.1\r\n
Host: whatever\r\n
Test: 1\r\n
 2\r\n
 3\r\n
\r\n

...it sees the Test header as having a value of 3, instead of 1 2 3.

❓ What is the motivation / use case for changing the behavior?
This is a logic bug.

💡 To Reproduce
Steps to reproduce the behavior:

1. Run a Cheroot server that echos back received headers (such as this one).
2. Send it a request with field line folding:

printf 'GET / HTTP/1.1\r\nHost: whatever\r\nTest: a\r\n b\r\n c\r\n\r\n' \
    | timeout 1 nc 172.18.0.19 80 \
    | grep '"headers"' \
    | jq '.["headers"][1][1]' \
    | xargs echo \
    | base64 -d \
    | xxd

3. See the extracted header value:

00000000: 63                                       c

💡 Expected behavior
Cheroot should either have a header value of a b c, a b c, or reject the request.

📋 Environment

• Cheroot version: 10.0.2.dev71+g1ff20b18
• Python version: 3.11.9
• OS: Linux 3dd8401f8901 6.9.7-arch1-1 #1 SMP PREEMPT_DYNAMIC Fri, 28 Jun 2024 04:32:50 +0000 x86_64 GNU/Linux