parser: Strip whitespace from references

parser: Strip whitespace from references

Some mail, particularly those generated with older versions of
git-send-email or written by hand, include some extra whitespace in the
'References' and 'In-Reply-To' lines. Ensure we always strip this,
preventing mismatches between this and 'Message-ID', which is already
stripped of whitespace, when looking up SeriesReference's.

Signed-off-by: Stephen Finucane <stephen@that.guru>