Fixed issue where virtualAttributes were not being applied to discriminated child entities when loading through the parent

[ryanalbrecht]

If you add a virtual attribute to a parent entity and load a discriminated child entity through that parent entity. The virtual attributes will not be applied to the child entity as quick will load a new instance of the child class and then use hydrate() to populate the entity. I fixed this by checking if the parent entity has any virtual attributes and then manually calling appendVirtualAttribute on the child class.

Additionally, thinking in terms of performance I cached the key name of all virtual entities in a new property called _virtualAttributes. This would eliminate the overhead (albeit a tiny one) of looping through all the attributes in the parent entity to check if it is a virtual attribute.