Update header docs, allow a blank subject.
Spelling fix, made header test more appropriate.
check for CRLF and not /$/ as line ending
update changelog
allow empty subj through header not subject
v0.008

        - correct the documentation of header-setting methods to match their
          behavior (since Email::Stuff's first release)

        - add tests to show we can set an empty subject through ->header
          but not through ->subject