minor: cleanup thread_to_socket pruning code and test

lib/mongo/util/pool.rb

@@ -227,15 +227,10 @@ module Mongo
     end
 
     def prune_thread_socket_hash
-      map = {}
-      Thread.list.each do |t|
-        map[t] = 1
-      end
+      current_threads = Set[*Thread.list]
 
-      @threads_to_sockets.keys.each do |key|
-        if !map[key]
-          @threads_to_sockets.delete(key)
-        end
+      @threads_to_sockets.delete_if do |thread, socket|
+        !current_threads.include?(thread)
       end
     end
 

test/pool_test.rb

@@ -12,12 +12,10 @@ class PoolTest < Test::Unit::TestCase
     @pool = Pool.new(@connection, TEST_HOST, TEST_PORT, :size => 5)
 
     @threads    = []
-    @sockets    = []
 
     10.times do
       @threads << Thread.new do
         original_socket = @pool.checkout
-        @sockets << original_socket
         @pool.checkin(original_socket)
         5000.times do
           socket = @pool.checkout