Silence uninitialized variable warnings

As the community progressively moves to a useful practice of enabling
ruby warnings on tests, assigning an instance variable before use becomes
a necessary practice. Here we set some variables at initialization that
were previously lazily or conditionally set:

- `decoded` is assigned false which seems to make more semantic sense
  than than using nil
- `namespace` is assigned nil, its value being lazily computed later
- `available_namespaces` is assigned nil so as to respect the cache
  invalidation mechanism

lib/oga/xml/attribute.rb

@@ -37,7 +37,8 @@ module Oga
         @name           = options[:name]
         @value          = options[:value]
         @element        = options[:element]
-
+        @decoded        = false
+        @namespace      = nil
         @namespace_name = options[:namespace_name]
       end
 

lib/oga/xml/element.rb

@@ -38,6 +38,7 @@ module Oga
         @namespace_name       = options[:namespace_name]
         @attributes           = options[:attributes] || []
         @namespaces           = options[:namespaces] || {}
+        @available_namespaces = nil
 
         link_attributes
         register_namespaces_from_attributes