Node#tag_name should return lower case

spec/driver_spec.rb

@@ -127,6 +127,6 @@ describe Capybara::Driver::Webkit do
   end
 
   it "returns a node's tag name" do
-    subject.find("//p").first.tag_name.should == "P"
+    subject.find("//p").first.tag_name.should == "p"
   end
 end

src/capybara.js

@@ -27,7 +27,7 @@ Capybara = {
   },
 
   tagName: function(index) {
-    return this.nodes[index].tagName;
+    return this.nodes[index].tagName.toLowerCase();
   }
 };