Semantic blocks are not enforced in hash values

[ferdynton]

Hi! I applied standardrb to a project (love it ❤️) and I noticed this code didn't get corrected with the semantic block cop:

{
  foo: [].map do
    nil
  end,
}

Should this case be supported? Thanks!

[searls]

Since we've adopted the code for this (and only this) cop, feel free to take a stab at adding a test and fixing it!