Merge child class fillable traits with those of the parent class

[gcavanunez]

Highlights

• merges fillable arrays on child with parent fillable
• add dedicated tests for this behavior

References

• [Feature request] Merge fillable columns with parent #118
• Use parent's fillables in children models #77
• [#77] Use parent's fillables in children models #78

[mattstauffer]

Great code; just a few code style notes, which I think we can solve in the future by running Duster on the codebase and then using that for future code style.

[mattstauffer]

Suggested change

	* Merge the fillable attributes for the model with Parent Class
	* Merge the fillable attributes for the model with those of its Parent Class

[mattstauffer]

Delete this extra line please.