:swoosh_header removes custom email headers.
A Swoosh.Email.header(email, name, value) call collapses to the email (a piped stage to
what was piped into it). In Swoosh.Email.new/1, a headers: option with a literal map (or keyword) value has
one entry dropped per mutant; for an opaque value (a variable, a call), the whole headers:
option is dropped instead. An empty literal headers: value produces no mutations — removing
nothing is a no-op.
A survivor means no test asserts the header — precisely the tracking, list-management, and provider-routing headers that are set and never checked.