Bug#996784: deb822-repro: Doesn't preserve field name casing

[Jelmer Vernooij]

Package: python3-debian
Version: 0.1.39
Severity: normal
X-Debbugs-Cc: nthykier at debian.org

When updating a field using the deb822 repro code, the existing field name
casing is discarded and instead overriden by the user-provided casing. This is
inconsistent with the way that the current code works, and makes it harder
to make minimal changes to the existing file.

>>> from debian._deb822_repro.parsing import parse_deb822_file
>>> z = parse_deb822_file(["Source: blah", "Standards-version: 3.1.3"])
>>> list(z.iter_parts())[0]['Standards-Version'] = '1.2.3.4'
>>> z.convert_to_text()
'Source: blah\nStandards-Version: 1.2.3.4\n'

-- System Information:
Debian Release: bookworm/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 5.10.0-8-amd64 (SMP w/2 CPU threads)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages python3-debian depends on:
ii  python3          3.9.2-3
ii  python3-chardet  4.0.0-1
ii  python3-six      1.16.0-2

Versions of packages python3-debian recommends:
ii  python3-apt  2.2.1

Versions of packages python3-debian suggests:
ii  gpgv  2.2.27-2

-- no debconf information