gh-157880: Write visible headers of BabylMessage in mailbox.Babyl

[vasiliyk]

When adding a BabylMessage, Babyl._install_message() flattens its visible headers into a BytesIO and copies them line by line into the mailbox file. The buffer was never rewound after flattening, so readline() returned nothing and the visible headers block and the blank line after it were skipped.
Reading the message back with get_message() or get_bytes() then consumed the body as headers and failed with AssertionError.

Rewind the buffer before reading it, and add a test that adds a BabylMessage with visible headers and reads it back.

• Issue: mailbox.Babyl never writes the visible headers of a BabylMessage #157880