FIX: prevent Env#setup to override unused options

Resque::Scheduler::Env#setup was setting every possible option with a
default value, thus overriding a previously configured one.

For example, suppose you set Resque::Scheduler.dynamic to true, then
instantiating an Env with no options will reset the dynamic setting to
false. This is exactly what is described in #369
@yaauie sure! Thank you for the feedback. I'll refactor the code in that way.