Move valid_doc method to BulkWritable module and account for Chunk serialization

Author: estolfo

lib/mongo/bulk_write/bulk_writable.rb

@@ -85,6 +85,11 @@ def execute
 
       private
 
+      def valid_doc?(doc)
+        doc.respond_to?(:keys) ||
+          doc.respond_to?(:document)
+      end
+
       def write_concern
         @write_concern ||= WriteConcern.get(@options[:write_concern]) ||
                             @collection.write_concern

lib/mongo/bulk_write/deletable.rb

@@ -22,10 +22,6 @@ module Deletable
 
       private
 
-      def valid_doc?(doc)
-        doc.respond_to?(:keys)
-      end
-
       def validate_delete_op!(type, d)
         raise Error::InvalidBulkOperation.new(type, d) unless valid_doc?(d)
       end

lib/mongo/bulk_write/insertable.rb

@@ -22,10 +22,6 @@ module Insertable
 
       private
 
-      def valid_doc?(doc)
-        doc.respond_to?(:keys)
-      end
-
       def validate_insert_ops!(type, inserts)
         if inserts.empty?
           raise Error::InvalidBulkOperation.new(type, inserts)