mssql: don't encode contents when uploading attachments

Downloads are broken and inline preview doesn't work.

Move "case 'mssqlnative':" down next to 'oci8' in
db_prepare_binary_string (database_api) to effectively
return the string unchanged via 'default:'. Adjust comment.

Fixes 0022208

Signed-off-by: Damien Regad dregad@mantisbt.org