Synchronize around modifying hash concurrently

This sometimes caused a spec failure on Rubinius. The problem is that
the hash is modified in multiple threads simultanuously. This means that
an entry could get lost because of code executed concurrently.

This means we have to lock around this to do this safely.

Author: dbussink

spec/mysql2/client_spec.rb

@@ -293,13 +293,17 @@ def connect *args
 
       it "threaded queries should be supported" do
         threads, results = [], {}
+        lock = Mutex.new
         connect = lambda{
           Mysql2::Client.new(DatabaseCredentials['root'])
         }
         Timeout.timeout(0.7) do
           5.times {
             threads << Thread.new do
-              results[Thread.current.object_id] = connect.call.query("SELECT sleep(0.5) as result")
+              result = connect.call.query("SELECT sleep(0.5) as result")
+              lock.synchronize do
+                results[Thread.current.object_id] = result
+              end
             end
           }
         end