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
Merge branch 'fix-options-setting' of https://github.com/bugant/resque-scheduler into bugant-fix-options-setting
A few rubocop adjustments
Merge branch 'bugant-fix-options-setting'