instance_eval behaves different in Ruby 1.8 and Ruby 1.9, use instance_exec instead

In Ruby 1.9, instance_eval calls the block the with receiver as the first argument:

• In Ruby 1.8, receiver.instance_eval(&block) calls block.call()
• In Ruby 1.9, receiver.instance_eval(&block) calls block.call(receiver)

This will blow up in your face in Ruby 1.9, where a lambda crashes when it is called with a different number of arguments:

Copywrong number of arguments (1 for 0) (ArgumentError)

Forget that instance_eval ever existed. Use instance_exec instead, which behaves consistently across all Rubies.