Fix #305: presenting arrays of objects when the entity class isn't explicitly passed into present

[brandonweiss]

I wrote two failing specs and then refactored the code a little to make them pass. I tried to mirror the formatting as much as possible.

I had to manually create a named class in one place in order to properly create a failing test. Using Object wouldn't work because of the way present walks up the ancestor tree.

I also made an explicit choice to derive to auto-detect the entity from the first object in the collection because it was the simplest solution. It might make sense to expand it to auto-detect the entity of each object in the collection, in case someone returns a collection with different types of objects.

[dblock]

Merging. I think we can do a somewhat simpler logic around dealing with the Array, I'll take a quick stab at this.

[dblock]

I committed d3910e4, all I did is assign the object instance used to derive the Entity class and reuse it. This removes the need for the second ||=. Double-check after me, please.