Updated shape to use style method

Hi again all!

This one involved a little more tweaking than the other ones. It looks like the configuration specs were using Shape (randomly?) so I had to update them to not look for a style hash since shape doesn't use it.

Tests pass and everything else was straight forward so I think this is ready to go. 

PS. I'll merge this (and others) after at least one reviewer gives me the go ahead as per (1) [here](https://github.com/shoes/shoes4/wiki/Merging-a-Pull-Request).
When the `key` was `:fill` and the fill was a gradient, then `@style[key]` returned `Shoes::Gradient` so if you swapped it out for another gradient, the old `!=` would return `false` and fail to update the style.

I used `inspect` but is there a better solution?