Prevent errors in merge rulesets()

If the 'saving' $key is not set on the Model.

It was throwing a "array_merge(): Argument #1 is not an array" error on #204
Merge pull request #56 from tobz-nz/master

Prevent errors in merge rulesets()
Merge branch 'master' of https://github.com/dwightwatson/validating into develop