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

Should now work for instances having class member neither null
nor instanceof Class, as long as getClass() returns a Class instance.

See https://jira.codehaus.org/browse/GROOVY-6387

Signed-off-by: Thibault Kruse <kruset@in.tum.de>
useless function call
more code style, using class member instead of getClass Method