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

@@ -34,10 +34,11 @@ module Oga
       # @option options [String] :value
       # @option options [Oga::XML::Element] :element
       def initialize(options = {})
-        @name    = options[:name]
-        @value   = options[:value]
-        @element = options[:element]
-
+        @name           = options[:name]
+        @value          = options[:value]
+        @element        = options[:element]
+        @decoded        = false
+        @namespace      = nil
         @namespace_name = options[:namespace_name]
       end
 

lib/oga/xml/element.rb

@@ -34,10 +34,11 @@ module Oga
       def initialize(options = {})
         super
 
-        @name           = options[:name]
-        @namespace_name = options[:namespace_name]
-        @attributes     = options[:attributes] || []
-        @namespaces     = options[:namespaces] || {}
+        @name                 = options[:name]
+        @namespace_name       = options[:namespace_name]
+        @attributes           = options[:attributes] || []
+        @namespaces           = options[:namespaces] || {}
+        @available_namespaces = nil
 
         link_attributes
         register_namespaces_from_attributes