fix and implement missing resource controller specs

# Commit 1
The old 2 skipped tests are failing while this overwrites the rspec `post` method:
```ruby
let(:post) { Post.new title: "An incledibly unique Post Title" }
```

To use a `post` request in a unit spec is not a good choice, while with post you can't expect a internal rised error, you can only expect the external error, so I change it.

# Commit 2
This is not the best way, but it looks like this is the only one and we don't need to test the right behavior of `instance_exec`.