Include "TRUE" string in SerializationHelper::convert_boolean

I'm currently testing H2 to MySQL migration. Works fine, except the boolean values are all false when migrating from H2 to MySQL.
The reason is simple. H2 databases write boolean values as strings with capital letters (e.g "TRUE").

Adding the "TRUE" String to the ::convert_boolean method fixes this problem.

Signed-off-by: Jason Franklin <franklin@equinux.com>