Generified AbstractConverter.

AbstractConverter now implements the generified convert() method. It now has a
generic parameter for its default type. This commit also fixes
[BEANUTILS-445].

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/beanutils/branches/java5@1533966 13f79535-47bb-0310-9956-ffa450edef68