Rails 5.0.1 breaks the behaviour of .group(enum).count with PostgreSQL adapter

enum_group_issue.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  # If you change the version to "5.0.0.1", the test will pass.
  # `git bisect` says that 9c28c54879 is the first bad commit.
  gem "activerecord", "5.0.1"
  gem "pg"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection adapter: "postgresql"
ActiveRecord::Base.connection.drop_database "test_group_enum"
ActiveRecord::Base.connection.create_database "test_group_enum"
ActiveRecord::Base.establish_connection adapter: "postgresql", database: "test_group_enum"
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.integer :category
  end
end

class Post < ActiveRecord::Base
  enum category: %i(sport entertainment)
end

class BugTest < Minitest::Test
  def test_association_stuff
    Post.create! category: 'sport'
    Post.create! category: 'entertainment'

    # Up to Rails 5.0.0.1, this was the result:
    assert_equal Post.group(:category).count, {1=>1, 0=>1}

    # Since Rails 5.0.1 this is the result instead:
    # assert_equal Post.group(:category).count, {"sport"=>1, "entertainment"=>1}
    # `git bisect` says that 9c28c54879 is the first bad commit.
  end
end