Only execute update if document attributes is not empty

Author: Jon Whiteaker

elasticsearch-model/lib/elasticsearch/model/indexing.rb

@@ -426,7 +426,7 @@ def update_document(options={})
                 type:  document_type,
                 id:    self.id,
                 body:  { doc: attributes } }.merge(options)
-            )
+            ) unless attributes.empty?
           else
             index_document(options)
           end