Removed Document#available_namespaces.

Namespaces aren't scoped per document but instead per element, thus this method
doesn't make that much sense. This also fixes the remaining, failing XPath test.

lib/oga/xml/document.rb

@@ -89,23 +89,6 @@ module Oga
         end
       end
 
-      ##
-      # Returns the available namespaces. These namespaces are retrieved from
-      # the first element in the document.
-      #
-      # @see [Oga::XML::Element#available_namespaces]
-      #
-      def available_namespaces
-        children.each do |child|
-          # There's no guarantee that the first node is *always* an element
-          # node.
-          return child.available_namespaces if child.is_a?(Element)
-        end
-
-        # In case the document is empty.
-        return {}
-      end
-
       ##
       # Converts the document and its child nodes to XML.
       #

lib/oga/xpath/evaluator.rb

@@ -411,6 +411,8 @@ module Oga
         name  = ast_node.children[1]
 
         context.each do |context_node|
+          next unless context_node.respond_to?(:available_namespaces)
+
           context_node.available_namespaces.each do |_, namespace|
             if namespace.name == name or name == '*'
               nodes << namespace

spec/oga/xml/document_spec.rb

@@ -71,20 +71,6 @@ describe Oga::XML::Document do
     end
   end
 
-  context '#available_namespaces' do
-    example 'return an empty Hash by default' do
-      described_class.new.available_namespaces.should be_empty
-    end
-
-    example 'return the namespaces of the first element' do
-      namespace = Oga::XML::Namespace.new(:name => 'x')
-      element   = Oga::XML::Element.new(:namespaces => {'x' => namespace})
-      document  = described_class.new(:children => [element])
-
-      document.available_namespaces['x'].should == namespace
-    end
-  end
-
   context '#to_xml' do
     before do
       child = Oga::XML::Comment.new(:text => 'foo')