Add Nokogiri::XML::Document#collect_namespaces_[href_keys|prefix_keys]

See sparklemotion#885 for details

lib/nokogiri/xml/document.rb

@@ -164,6 +164,48 @@ def collect_namespaces
         end
       end
 
+      ##
+      # In order to collect all namespaces in a document with multiple
+      # default namespace definitions use
+      #   - collect_namespaces_href_keys
+      #   - collect_namespaces_prefix_keys
+      #
+      # See https://github.com/sparklemotion/nokogiri/issues/885 for details
+      #
+      # collect_namespaces_href_keys will return e.g.
+      #
+      #   {
+      #     "http://www.w3.org/XML/1998/namespace"              =>  ["xml"],
+      #     "http://www.smpte-ra.org/schemas/429-7/2006/CPL"    =>  [nil],
+      #     "http://isdcf.com/schemas/draft/2011/cpl-metadata"  =>  ["meta"],
+      #     "http://www.w3.org/2000/09/xmldsig#"                =>  [nil]
+      #   }
+      #
+      # where nil implies xmlns.
+      #
+      # collect_namespaces_prefix_keys will return e.g.
+      #
+      #   {
+      #     "xml"   =>  ["http://www.w3.org/XML/1998/namespace"],
+      #     "nil"   =>  ["http://www.smpte-ra.org/schemas/429-7/2006/CPL", "http://www.w3.org/2000/09/xmldsig#"],
+      #     "meta"  =>  ["http://isdcf.com/schemas/draft/2011/cpl-metadata"]
+      #   }
+      #
+      # where nil implies xmlns.
+      #
+      def collect_namespaces_href_keys
+        xpath("//namespace::*").inject({}) do |hash, ns|
+          (hash[ ns.href ] ||= []) << ns.prefix unless (hash[ ns.href ] and hash[ ns.href ].include? ns.prefix)
+          hash
+        end
+      end
+      def collect_namespaces_prefix_keys
+        xpath("//namespace::*").inject({}) do |hash, ns|
+          (hash[ ns.prefix ] ||= []) << ns.href unless (hash[ ns.prefix ] and hash[ ns.prefix ].include? ns.href)
+          hash
+        end
+      end
+
       # Get the list of decorators given +key+
       def decorators key
         @decorators ||= Hash.new