Merge deep
Use Utils.deep_merge_hashes
Add a test
Ensure the post categories are unique (only one). Ref: #2527 #2531

h/t @Nicholas-Perry
https://github.com/jekyll/jekyll/issues/2527#issuecomment-47041756
Merge pull request #2490 from penibelst/deep-defaults
Update history to reflect merge of #2490 [ci skip]