Fix a bug in the get_msg_header() method introduced in rev 1.195 (12-Nov-2016):

The 'expand_fields' argument wasn't be parsed correctly and defaults to 'true',
so any script that modifies a message header could be writing-back expandeed
header fields which may sometimes fail depending on how much the total header
size expanded.
Enhancement: the put_msg_header() method will now fail with an exception if a
script tries to write-back a message header that was read with expanded fields.
This should help to avoid potential message base corruption through a script
bug.