Don't print empty attributes

This was a side-effect of #33, causing empty class lists in Active
Admin: https://travis-ci.org/activeadmin/active_admin/jobs/32816473

lib/arbre/html/attributes.rb

@@ -4,13 +4,22 @@ module HTML
     class Attributes < Hash
 
       def to_s
-        self.collect do |name, value|
+        map do |name, value|
+          next if value_empty?(value)
           "#{html_escape(name)}=\"#{html_escape(value)}\""
-        end.join(" ")
+        end.compact.join ' '
+      end
+
+      def any?
+        super{ |k,v| !value_empty?(v) }
       end
 
       protected
 
+      def value_empty?(value)
+        value.respond_to?(:empty?) ? value.empty? : !value
+      end
+
       def html_escape(s)
         ERB::Util.html_escape(s)
       end

lib/arbre/html/tag.rb

@@ -141,9 +141,8 @@ def child_is_text?
         children.size == 1 && children.first.is_a?(TextNode)
       end
 
-
       def attributes_html
-        attributes.any? ? " " + attributes.to_s : nil
+        " #{attributes}" if attributes.any?
       end
 
       def set_for_attribute(record)

spec/arbre/unit/html/tag_attributes_spec.rb

@@ -13,9 +13,15 @@
     end
 
     it "should render the attributes to html" do
-      expect(tag.to_s).to eq <<-HTML
-<tag id="my_id"></tag>
-HTML
+      expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
+    end
+
+    it "shouldn't render attributes that are empty" do
+      tag.class_list # initializes an empty ClassList
+      tag.set_attribute :foo, ''
+      tag.set_attribute :bar, nil
+
+      expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
     end
 
     it "should get an attribute value" do
@@ -45,16 +51,12 @@
 
   describe "rendering attributes" do
     it "should html safe the attribute values" do
-      tag.set_attribute(:class, "\">bad things!")
-      expect(tag.to_s).to eq <<-HTML
-<tag class="&quot;&gt;bad things!"></tag>
-HTML
+      tag.set_attribute(:class, '">bad things!')
+      expect(tag.to_s).to eq "<tag class=\"&quot;&gt;bad things!\"></tag>\n"
     end
     it "should should escape the attribute names" do
       tag.set_attribute(">bad", "things")
-      expect(tag.to_s).to eq <<-HTML
-<tag &gt;bad="things"></tag>
-HTML
+      expect(tag.to_s).to eq "<tag &gt;bad=\"things\"></tag>\n"
     end
   end
 end