fix 6387: groovysh completion for maps with 'class' entries

Should now work for instances having class member neither null
nor instanceof Class, as long as getClass() returns a Class instance.
E.g. map = ['class': 42]

See https://jira.codehaus.org/browse/GROOVY-6387


Signed-off-by: Thibault Kruse <kruset@in.tum.de>