Don't modify raw namespaces.

When calling Element#available_namespaces the list of namespaces returned by
Element#namespaces must not be modified.

lib/oga/xml/element.rb

@@ -273,7 +273,7 @@ module Oga
       # @return [Hash]
       #
       def available_namespaces
-        merged = namespaces
+        merged = namespaces.dup
         node   = parent
 
         while node && node.respond_to?(:namespaces)

spec/oga/xml/element_spec.rb

@@ -418,6 +418,10 @@ describe Oga::XML::Element do
     example 'return the "baz" namespace for the parent' do
       @parent_ns['baz'].uri.should == 'yyy'
     end
+
+    example 'do not modify the list of direct namespaces' do
+      @child.namespaces.key?('foo').should == false
+    end
   end
 
   context '#self_closing?' do