Allow resolving parameters using resolve values.

lib/public_activity/common.rb

@@ -247,10 +247,11 @@ def prepare_custom_fields(options)
     # be serialized into the Activity#parameters column
     # @api private
     def prepare_parameters(parameters)
-      params = {}
-      params.merge!(self.class.activity_parameters_global)
-        .merge!(parameters || {})
-      params.each { |k, v| params[k] = PublicActivity.resolve_value(self, v) }
+      parameters ||= {}
+
+      [self.class.activity_parameters_global, parameters].reduce({}) do |params, value|
+        params.merge!(PublicActivity.resolve_value(self, value))
+      end
     end
 
     # Prepares relation to be saved

test/test_common.rb

@@ -71,6 +71,15 @@
     subject.activities.last.nonstandard.must_equal "true"
   end
 
+  it 'allows resolving parameters' do
+    subject.save
+    subject.expects(:custom_parameters).returns({ type: 'News' })
+
+    subject.create_activity :update, parameters: :custom_parameters
+
+    subject.activities.last.parameters[:type].must_equal 'News'
+  end
+
   it 'accepts owner as a symbol' do
     klass = article(:owner => :user)
     @article = klass.new(:user => @owner)