Enforce symbols for element attributes.

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

lib/oga/xml/element.rb

@@ -41,7 +41,7 @@ module Oga
       # @return [String]
       #
       def attribute(name)
-        return attributes[name]
+        return attributes[name.to_sym]
       end
 
       alias_method :attr, :attribute

lib/oga/xml/parser.y

@@ -137,10 +137,10 @@ rule
 
   attribute
     # foo
-    : T_ATTR { {val[0] => nil} }
+    : T_ATTR { {val[0].to_sym => nil} }
 
     # foo="bar"
-    | T_ATTR T_STRING { {val[0] => val[1]} }
+    | T_ATTR T_STRING { {val[0].to_sym => val[1]} }
     ;
 
   # XML declarations