Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

lib/active_model/dates.rb

@@ -1,5 +1,4 @@
 module ActiveModel::Dates
-
   def parse_date(field, attrs)
     year, month, day = attrs["#{field}(1i)"],
                        attrs["#{field}(2i)"],
@@ -12,5 +11,4 @@ module ActiveModel::Dates
   def remove_date(field, attrs)
     attrs.except("#{field}(1i)", "#{field}(2i)", "#{field}(3i)")
   end
-
 end