MCO-156 - don't set environment variables with the value of nil

Here we change the behaviour of the Shell helper filter the setting of any
environment variables with the value nil.

The initial usecase is the ability to inherit LC_ALL from the environment of
the mcollectived, to achieve this the user would use something like this:

  Shell.new('date', :environment => { 'LC_ALL' => nil })
Merge pull request #224 from richardc/MCO-156-LC_ALL

MCO-156 - don't set environment variables with the value of nil