re-add #asset_url helper (removed in 12fed9a)

fixes #810

app/helpers/blacklight/url_helper_behavior.rb

@@ -215,4 +215,9 @@ def remove_facet_params(field, item, source_params=params)
     p
   end
 
-end
+  if ::Rails.version < "4.0"
+    def asset_url *args
+      "#{request.protocol}#{request.host_with_port}#{asset_path(*args)}"
+    end
+  end
+end

spec/views/catalog/opensearch.xml.builder_spec.rb

@@ -0,0 +1,10 @@
+require 'spec_helper'
+
+describe 'catalog/opensearch.xml.builder' do
+  it "should render an OpenSearch description document" do
+    render
+    doc = Nokogiri::XML.parse(rendered)
+    doc.remove_namespaces!
+    expect(doc.xpath('/OpenSearchDescription').length).to eq 1
+  end
+end