prevent object representations from including properties without valu…

…es (fixes digitalocean#4)

lib/kartograph/artist.rb

@@ -15,8 +15,10 @@ def draw(scope = nil)
       scoped_properties = scope ? properties.filter_by_scope(scope) : properties
       scoped_properties.each_with_object({}) do |property, mapped|
         raise ArgumentError, "#{object} does not respond to #{property.name}, so we can't map it" unless object.respond_to?(property.name)
-        mapped[property.key] = property.value_for(object, scope)
+        if value = property.value_for(object, scope)
+          mapped[property.key] = value
+        end
       end
     end
   end
-end
+end

spec/lib/kartograph/artist_spec.rb

@@ -27,6 +27,16 @@
       expect(masterpiece).to include(hello: 'world')
     end
 
+    it 'does not return a key if the value is not set' do
+      object     = double('object', hello: nil)
+      properties << Kartograph::Property.new(:hello)
+
+      artist = Kartograph::Artist.new(object, map)
+      masterpiece = artist.draw
+
+      expect(masterpiece).not_to include(:hello)
+    end
+
     it 'raises for a property that the object does not have' do
       object = double('object')
       properties << Kartograph::Property.new(:bunk)
@@ -79,4 +89,4 @@
       end
     end
   end
-end
+end