Add test for merging associative properties.

Associated/hash properties should be merged like
dictionaries/array_merge.

Refs #2914
Do not merge the contents of merged properties.

Merging the contents/configuration in merged properties often results in
the wrong answer as non-associative values will be duplicated N times
where N is the number of ancestors with the same value. Instead, only
merge missing top level items. The config for each item will be taken
from the top most class it is defined in.

Refs #2914
Merge pull request #2922 from markstory/issue-2914

3.0 remove deep merging from mergeVars