Replace .lstrip('\n') with .removeprefix("\n") to preserve body text that
legitimately starts with blank lines. The serialize() function inserts exactly
one separator newline; removing only that one newline (via removeprefix) rather
than all leading newlines (via lstrip) maintains parse/serialize roundtrip
losslessness.
Adds regression test to verify bodies with leading blank lines roundtrip
correctly.
rootiest
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