Returns a hash that represents the difference between two hashes.

Examples:

  {1 => 2}.diff(1 => 2)         # => {}
  {1 => 2}.diff(1 => 3)         # => {1 => 2}
  {}.diff(1 => 2)               # => {1 => 2}
  {1 => 2, 3 => 4}.diff(1 => 2) # => {3 => 4}