stringify facts, if required, before collecting custom facts

This is the result of some poking in
voxpupuli/voxpupuli-test#131 and
voxpupuli/voxpupuli-test#132 (comment)

Tested in: voxpupuli/puppet-example#54

lib/rspec-puppet-facts.rb

@@ -170,11 +170,10 @@ def on_supported_os_implementation(opts = {})
       os = "#{facts[:operatingsystem].downcase}-#{operatingsystemmajrelease}-#{facts[:hardwaremodel]}"
       next unless os.start_with? RspecPuppetFacts.spec_facts_os_filter if RspecPuppetFacts.spec_facts_os_filter
       facts.merge! RspecPuppetFacts.common_facts
+      facts = stringify_keys(facts) if RSpec.configuration.facterdb_string_keys
       os_facts_hash[os] = RspecPuppetFacts.with_custom_facts(os, facts)
     end
 
-    return stringify_keys(os_facts_hash) if RSpec.configuration.facterdb_string_keys
-
     os_facts_hash
   end