Separate XML attributes by spaces.

This was originally reported by @jrochkind and partially patched by @billdueber.
My patches are built upon the latter, but without the need of using Array#map,
Array#join, etc. They also contain a few style changes.

This fixes #32 and #33.

lib/oga/xml/element.rb

@@ -211,11 +211,9 @@ module Oga
         attrs = ''
 
         attributes.each do |attr|
-          attrs << attr.to_xml
+          attrs << " #{attr.to_xml}"
         end
 
-        attrs = " #{attrs}" unless attrs.empty?
-
         return "<#{ns}#{name}#{attrs}>#{body}</#{ns}#{name}>"
       end
 

spec/oga/xml/element_spec.rb

@@ -261,7 +261,7 @@ describe Oga::XML::Element do
       instance.to_xml.should == '<foo:p></foo:p>'
     end
 
-    example 'include the attributes if present' do
+    example 'include a single attribute if present' do
       instance = described_class.new(
         :name       => 'p',
         :attributes => [
@@ -272,6 +272,18 @@ describe Oga::XML::Element do
       instance.to_xml.should == '<p key="value"></p>'
     end
 
+    example 'include multiple attributes if present' do
+      instance = described_class.new(
+        :name       => 'p',
+        :attributes => [
+          Oga::XML::Attribute.new(:name => 'key1', :value => 'value1'),
+          Oga::XML::Attribute.new(:name => 'key2', :value => 'value2'),
+        ]
+      )
+
+      instance.to_xml.should == '<p key1="value1" key2="value2"></p>'
+    end
+
     example 'include the child nodes if present' do
       instance = described_class.new(
         :name     => 'p',