keep speciale fields for bag-info.txt separate and refuse adding with method add_info

[nicolasfranck]

$bagit->add_file( IO::File->new("myfile"), "r" );
$bagit->add_info( "Bag-Size",[]);
$bagit->write("mydirectory");

Bag-Size now reads:

Bag-Size: ARRAY(0x8993)

Explanation:

• I added this special field Bag-Size after I added the file. This way Bag-Size is not overwritten, as _update_info is not triggered. If you do it in another order, then you won't get this situation.

• Of course I should not add these kind of fields, but shouldn't these keys be disallowed? The method remove_info for example refuses to remove them:

https://github.com/LibreCat/Catmandu-BagIt/blob/master/lib/Catmandu/BagIt.pm#L654

• Maybe keep these keys separate, outside of this info object, so no one has the ability to overwrite them accidentally as I did?