Use a builder to render changeset downloads

app/views/api/changesets/download.xml.builder

@@ -0,0 +1,19 @@
+xml.instruct! :xml, :version => "1.0"
+
+xml.osmChange(OSM::API.new.xml_root_attributes) do |osm|
+  @created.each do |elt|
+    osm.create do |create|
+      create << render(elt)
+    end
+  end
+  @modified.each do |elt|
+    osm.modify do |modify|
+      modify << render(elt)
+    end
+  end
+  @deleted.each do |elt|
+    osm.delete do |delete|
+      delete << render(elt)
+    end
+  end
+end