Fix subject and ref bug

[CTY-git]

PR Checklist

• The commit message follows our guidelines: Code of conduct
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Does this PR introduce a breaking change?
• Include PR in release notes?

PR Type

• Bugfix
• Feature
• Refactoring
• Build /CI
• Documentation
• Others

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

[patched-admin]

File Changed: patchwork/steps/SendEmail/SendEmail.py

Rule 1: Do not ignore potential bugs in the code
Details: Identified a bug correction rather than a bug introduction. The change fixes RFC compliance by correcting the email header from "Reference" to "References". This is actually a bug fix as the correct header name according to RFC 2822 is "References" (plural).

Affected Code Snippet:

msg.add_header("Reference", self.reply_message_id)  # Before
msg.add_header("References", self.reply_message_id) # After

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