InlineSchemaContainer: uncover a bug, add a failing test

Guava's ImmutableMap.builder() allows you to swallow duplicate keys but not to
build a map out of it. As a result, when a duplicate "id" is encountered, it
fails with an IllegalArgumentException -- which wasn't caught, since it is an
unchecked exception.

Add simple test to highlight this bug.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>