Optionally retain scope when evaluating procs in collections

This allows to receive the scope as the argument to a proc-value instead
of always executing the proc in the scope's context.

lib/rackstash/fields/abstract_collection.rb

@@ -104,7 +104,9 @@ module Rackstash
 
       def resolve_value(value, scope: nil)
         return value unless value.is_a?(Proc)
-        scope.nil? ? value.call : scope.instance_exec(&value)
+
+        return value.call if scope.nil?
+        value.arity == 0 ? scope.instance_exec(&value) : value.call(scope)
       rescue
         value.inspect
       end

spec/rackstash/fields/abstract_collection_spec.rb

@@ -261,9 +261,26 @@ describe Rackstash::Fields::AbstractCollection do
           expect(normalize(hash, scope: scope)['beep']).to be_a Rackstash::Fields::Hash
           expect(normalize(hash, scope: scope)['beep']['scope']).to eql 'SCOPE'
         end
+
+        it 'passes the supplied scope but retains self' do
+          scope = 'scope'
+          test_self = self
+
+          hash = {
+            beep: ->(v) {
+              expect(self).to equal test_self
+              { v.upcase => -> { self } }
+            }
+          }
+
+          expect(normalize(hash, scope: scope)).to be_a Rackstash::Fields::Hash
+          expect(normalize(hash, scope: scope)['beep']).to be_a Rackstash::Fields::Hash
+          expect(normalize(hash, scope: scope)['beep']['SCOPE']).to eql 'scope'
         end
       end
 
+    end
+
     describe 'with Array' do
       it 'wraps the array in a Rackstash::Fields::Array' do
         array = ['beep', 'boop']