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
This may be a little more clear, and easier to reason about, especially alongside the current setup:
~~~ ruby
options.key?(:app_name)   && c.app_name = options[:app_name]
options.key?(:dynamic)    && c.dynamic = !!options[:dynamic]
options.key?(:env)        && c.env = options[:env]
options.key?(:logfile)    && c.logfile = options[:logfile]
options.key?(:logformat)  && c.logformat = options[:logformat]
options.key?(:poll_sleep_amount) && c.poll_sleep_amount = Float(options[:poll_sleep_amount])
options.key?(:verbose)    && c.verbose = !!options[:verbose]
~~~

~~~