Use message-field-value instead of message-fetch-field in FCC header setup.

Use message-field-value instead of message-fetch-field in FCC header setup.

For message-fetch-field the buffer is expected to be narrowed to
just the header of the message.  That is not the case when
notmuch-fcc-header-setup is run, hence a wrong header value may be
returned.  E.g. when forwarding an
email, (message-fetch-field "From") returns the From header value
of the forwarded email.

Message-field-value is the same as message-fetch-field, only
narrows the buffer to the headers first.

Signed-off-by: Jameson Graef Rollins <jrollins@finestructure.net>