Remove a couple special cases

This removes a couple special cases to simplify things a bit more.

[`ad1a9ad4`](https://github.com/chrishunt/json-merge_patch/commit/ad1a9ad499d7599068857de062bf796f21d7086c) Removes the special case for `nil` since it's basically a primitive in Ruby anyway. Makes the code easier to read.

[`7c7362da`](https://github.com/chrishunt/json-merge_patch/commit/7c7362dab9edf9ed714fc05940e483c4e2c113cd) Removes the unnecessary check for a hash. We already know at this point that we're dealing with a hash.