show: Use consistent header ordering in the text format

show: Use consistent header ordering in the text format

Previously, top-level message headers were printed as Subject, From,
To, Date, while embedded message headers were printed From, To,
Subject, Date.  This makes both cases use the former order and updates
the tests accordingly.

Strangely, the raw format also uses this function, so this also fixes
the two raw format tests affected by this change.