     formatHeader :: String -> String -> ByteStringSource
Formats an e-mail header by encoding any non-ascii characters
using UTF-8 and Q-encoding, and folding lines at appropriate
points. It doesn't do more than that, so the header name and
header value should be well-formatted give or take line length
and encoding. So no non-ASCII characters within quoted-string,
quoted-pair, or atom; no semantically meaningful signs in names;
no non-ASCII characters in the header name; etcetera.