Enforce symbols for element attributes.

This comes with a little bit of memory overhead but this should be minor in
most cases.

spec/oga/xml/element_spec.rb

@@ -20,7 +20,7 @@ describe Oga::XML::Element do
 
   context '#attribute' do
     before do
-      @instance = described_class.new(:attributes => {'key' => 'value'})
+      @instance = described_class.new(:attributes => {:key => 'value'})
     end
 
     example 'return an attribute' do
@@ -42,7 +42,7 @@ describe Oga::XML::Element do
     example 'include the attributes if present' do
       instance = described_class.new(
         :name       => 'p',
-        :attributes => {'key' => 'value'}
+        :attributes => {:key => 'value'}
       )
 
       instance.to_xml.should == '<p key="value"></p>'

spec/oga/xml/parser/elements_spec.rb

@@ -43,7 +43,7 @@ describe Oga::XML::Parser do
     end
 
     example 'set the attributes of the element' do
-      @element.attributes.should == {'bar' => 'baz'}
+      @element.attributes.should == {:bar => 'baz'}
     end
   end
 

spec/oga/xml/parser/html_void_elements_spec.rb

@@ -39,7 +39,7 @@ describe Oga::XML::Parser do
     end
 
     example 'set the attributes' do
-      @node.attributes.should == {'href' => 'foo'}
+      @node.attributes.should == {:href => 'foo'}
     end
   end
 end