Make Sure Hash Keys are Symbolized

The internal implementation is relying on hashes with symbols as keys (e.g. [this part](https://github.com/flapjack/flapjack-diner/blob/master/lib/flapjack-diner/resources/notification_rules.rb#L41-L52)). Hence, external hash input should be symbolized by default.

Wasn't sure where to put tests for this since tools isn't tested directly. Suggestions?