Less html? calls in Element#available_namespaces

Previously it would always call the "html?" method, even if the
available namespaces were already set.

lib/oga/xml/element.rb

@@ -320,9 +320,10 @@ module Oga
       #
       def available_namespaces
         # HTML(5) completely ignores namespaces
+        unless @available_namespaces
           if html?
-          return @available_namespaces ||= {}
-        elsif !@available_namespaces
+            @available_namespaces = {}
+          else
             merged = namespaces.dup
             node   = parent
 
@@ -336,6 +337,7 @@ module Oga
 
             @available_namespaces = merged
           end
+        end
 
         return @available_namespaces
       end