Fix memory leak in development mode

Keying these hashes by klass causes reloadable classes to never get
freed. Thanks to @thedarkone for pointing this out in
the comments on 221571beb6b4bb7437989bdefaf421f993ab6002.

This doesn't seem to make a massive difference to performance.

Benchmark
---------

require 'active_record'
require 'benchmark/ips'

class Post < ActiveRecord::Base
  establish_connection adapter: 'sqlite3', database: ':memory:'
end

GC.disable

Benchmark.ips(20) do |r|
  r.report { Post.connection }
end

Before
------

Calculating -------------------------------------
                          5632 i/100ms
-------------------------------------------------
                       218671.0 (±1.9%) i/s -    4364800 in  19.969401s

After
-----

Calculating -------------------------------------
                          8743 i/100ms
-------------------------------------------------
                       206525.9 (±17.8%) i/s -    4039266 in  19.992590s

0 files changed, 0 insertions, 0 deletions